So the default behavior was changed in 1.21, however, there is no way to override the `network-bridge: lxcbr0` setting now. For the installer this is a breaking change and imo a regression.

My environments.yaml file:

default: local

environments:
  local:
    type: local
    container: kvm
    lxc-clone: true
    authorized-keys-path: ~/.ssh/id_rsa.pub
    default-series: trusty
    admin-secret: pass
    network-bridge: lxcbr0


But what actually happens is:

user: admin
password: pass
environ-uuid: 35f2fdbd-e6e5-45f2-895a-c621b006765f
state-servers:
- localhost:17070
- 192.168.122.1:17070
server-hostnames:
- localhost:17070
- 192.168.122.1:17070
ca-cert: '-----BEGIN CERTIFICATE-----

  MIICWzCCAcagAwIBAgIBADALBgkqhkiG9w0BAQUwQzENMAsGA1UEChMEanVqdTEy

  MDAGA1UEAwwpanVqdS1nZW5lcmF0ZWQgQ0EgZm9yIGVudmlyb25tZW50ICJsb2Nh

  bCIwHhcNMTUwMTIyMjIwNjIwWhcNMjUwMTI5MjIwNjE5WjBDMQ0wCwYDVQQKEwRq

  dWp1MTIwMAYDVQQDDClqdWp1LWdlbmVyYXRlZCBDQSBmb3IgZW52aXJvbm1lbnQg

  ImxvY2FsIjCBnzANBgkqhkiG9w0BAQEFAAOBjQAwgYkCgYEAuf1IP3/77Mq7ooXG

  Qo5RKD88xBNFQ3FjFXxPgyTF70AizzWu0bopu+MzEpTj3441yW5ZuXyshJqATx2h

  m5qNR8vYYV6fW5pXqsV0JaqMuutCm0T1+nf7auk+wGm8VJeBRDJrQae+olLAC748

  u+KIK2cpMNFVND265yPl+wykHTcCAwEAAaNjMGEwDgYDVR0PAQH/BAQDAgCkMA8G

  A1UdEwEB/wQFMAMBAf8wHQYDVR0OBBYEFKSIgnp7Wssc+hO8TMev187lMcEPMB8G

  A1UdIwQYMBaAFKSIgnp7Wssc+hO8TMev187lMcEPMAsGCSqGSIb3DQEBBQOBgQBo

  /j3rA/seqkRLgczm/mpdiHt47JZ8RNOHtBzF3z+GORJJxYfqUT2b96ZxCB5y8gNA

  VVGVXkTYGjCxRpRNv+iLieB8rMjI86Ehfl/w1S7KlT99W9tXjKKqRs2Nj1sOrqwn

  vEaCN5pHPRN1fQJwV1d0XVbCBQC9tqQAUs9O/Gxdig==

  -----END CERTIFICATE-----

'
bootstrap-config:
  admin-secret: pass
  agent-metadata-url: ""
  agent-version: 1.21.1
  api-port: 17070
  authorized-keys: 'ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAABAQDSS2RA47mMlT6SjBLfQaSLVMrkjB+lQn4rqG0KWVPl9LzrZZOVaHGsMI2So2vTELG3pP+omih8gJCiP3riwIQxn6aIEWOLyPBBVSzyq1kHItaCAB1SctadBnRSEtTV/lGhGIcuKIabiv/YzB1Q8YQ6ohUz6lGM2LPoiDD3fOxAUeylUPU7Q7R9nPcVHFIfpMLvkj4iWbIy3UpmCD7VJVVA5sfwwkS5VDjlaAgHABsxEkBqWqpiHOtONNbaOWdeKJJ0Qj8qKQulKyl3nz0OEynYQ6M9S1TrmSZW665cQOtIUQJ9oTRQRTNviiQ4D1+EiMq2wBYzvQHrf2I5rSvtgCDN
    juju-client-key

    ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAABAQDCBMeB+u4tCPtZ3Ul0UtvqvNxQvY765+gvqzNwX9V4EhtKtzPgnCFahk+NjMZBnsUZv5MHgyiF56gdB/NyKKJiQmNC/O5FG1GRvnOoFmEiPwinx6pihjbHKY4NMW9H8Rgqhv3Gcct4z++ItPJQvoLZpzVILTQBTevA/KDqgRlsmPFsfRzvSd14oykaOZBWMKYsF5qNQwWnofMp0KcUukWdWY0m/cCKXLIG/ChyDUtSXNFogNv/RZIg6u5i/9C3ShGG/L5nczDri28s3OuGVS79AcHWLKIN/QSuBBI4JdyLeFJzQwezqWFAS//qKPX/WBxHYRKB2FmcA1RgvoRR/Znv
    adam@maas

'
  bootstrap-addresses-delay: 10
  bootstrap-retry-delay: 5
  bootstrap-timeout: 600
  ca-cert: '-----BEGIN CERTIFICATE-----

    MIICWzCCAcagAwIBAgIBADALBgkqhkiG9w0BAQUwQzENMAsGA1UEChMEanVqdTEy

    MDAGA1UEAwwpanVqdS1nZW5lcmF0ZWQgQ0EgZm9yIGVudmlyb25tZW50ICJsb2Nh

    bCIwHhcNMTUwMTIyMjIwNjIwWhcNMjUwMTI5MjIwNjE5WjBDMQ0wCwYDVQQKEwRq

    dWp1MTIwMAYDVQQDDClqdWp1LWdlbmVyYXRlZCBDQSBmb3IgZW52aXJvbm1lbnQg

    ImxvY2FsIjCBnzANBgkqhkiG9w0BAQEFAAOBjQAwgYkCgYEAuf1IP3/77Mq7ooXG

    Qo5RKD88xBNFQ3FjFXxPgyTF70AizzWu0bopu+MzEpTj3441yW5ZuXyshJqATx2h

    m5qNR8vYYV6fW5pXqsV0JaqMuutCm0T1+nf7auk+wGm8VJeBRDJrQae+olLAC748

    u+KIK2cpMNFVND265yPl+wykHTcCAwEAAaNjMGEwDgYDVR0PAQH/BAQDAgCkMA8G

    A1UdEwEB/wQFMAMBAf8wHQYDVR0OBBYEFKSIgnp7Wssc+hO8TMev187lMcEPMB8G

    A1UdIwQYMBaAFKSIgnp7Wssc+hO8TMev187lMcEPMAsGCSqGSIb3DQEBBQOBgQBo

    /j3rA/seqkRLgczm/mpdiHt47JZ8RNOHtBzF3z+GORJJxYfqUT2b96ZxCB5y8gNA

    VVGVXkTYGjCxRpRNv+iLieB8rMjI86Ehfl/w1S7KlT99W9tXjKKqRs2Nj1sOrqwn

    vEaCN5pHPRN1fQJwV1d0XVbCBQC9tqQAUs9O/Gxdig==

    -----END CERTIFICATE-----

'
  ca-private-key: '-----BEGIN RSA PRIVATE KEY-----

    MIICXgIBAAKBgQC5/Ug/f/vsyruihcZCjlEoPzzEE0VDcWMVfE+DJMXvQCLPNa7R

    uim74zMSlOPfjjXJblm5fKyEmoBPHaGbmo1Hy9hhXp9bmleqxXQlqoy660KbRPX6

    d/tq6T7AabxUl4FEMmtBp76iUsALvjy74ogrZykw0VU0PbrnI+X7DKQdNwIDAQAB

    AoGAIfP3CICYwPWRo23Ab84DStGgijrqXhGJARccfNhtuet/b0xhia/uhnpHjJDZ

    uKoBb7BReOclp199JPRH2FwyhiUoB/KiA1drp1Rwb3zVhT6/+Bco67TfUEy3HnHY

    JbUUQlqYi55omb0brg4yIz75rHe/0hh/W+8jeL5fLJzBk2ECQQDcd4f3AzyN5hmn

    U7oZKKCq1qoAXdl634GBMKX5Al/g6+uMbwr9BWN0vPnJDnMjHnZhLQjfQ9rdFq57

    +8ho+dTFAkEA1/c1Ga5PSm6u/JSRSTeNG5eIWmXpZwFEpRfg5zaVrpTYSagzx3op

    mrUzeAs/OTAd7Shd3Ev4F/z7E8KXYV8hywJBAIlQvdPBJU8wWRwaDxgqcN/XMi1m

    RtW8Lho8JbQRoYwjBSQD6bEsUQTdyypKILZn+82M0jDVP5Vi0iyQ5wNTUA0CQQCh

    60Z0qqvoMnQgC4OiUXQKa+JfZGlKQTkmTnDg6Zvv+Mjw8Jthw8XNqG2eY1BToC/P

    z6QPtyWbVqEof4zgBGNhAkEAt1iKmFDbHNrdhu57Gbgautx0h1bPWRhd+pjMkagp

    mQ7Ol1NdAWjXAv7ykUoWLNjeMi4xK4jIHSY4halj1/7Gkw==

    -----END RSA PRIVATE KEY-----

'
  charm-store-auth: ""
  container: kvm
  default-series: trusty
  development: false
  disable-network-management: false
  firewall-mode: instance
  image-metadata-url: ""
  image-stream: ""
  logging-config: <root>=WARNING;unit=DEBUG
  lxc-clone: true
  lxc-clone-aufs: false
  name: local
  namespace: ubuntu-local
  network-bridge: virbr0  ### <- still virbr0?!?
  prefer-ipv6: false
  proxy-ssh: false
  root-dir: /home/ubuntu/.cloud-install/juju/local
  set-numa-control-policy: false
  ssl-hostname-verification: true
  state-port: 37017
  storage-port: 8040
  syslog-port: 6514
  test-mode: false
  tools-metadata-url: ""
  type: local
  uuid: 35f2fdbd-e6e5-45f2-895a-c621b006765f

I understand why the change was made, however, not sure why I can no longer override it?