configure the microk8s proxy

[Andrea Ieri]

In some environments pulling images directly from registries can be challenging. Given that the microk8s charm allows specifying proxies via the containerd_env charm option, the sunbeam cli could ask the user for a proxy host during the bootstrapping stage and automate configuring microk8s accordingly.

[Samuel Allan (samuelallan)]

To add more context and an example, this means that deploying on an instance in serverstack (as we do for testing) fails at the microk8s stage. We can workaround for now by running this during this stage:

```
juju config -m admin/controller microk8s containerd_env="\
# This file is managed by Juju. Manual changes may be lost at any time.
# Configure limits for locked memory and maximum number of open files
ulimit -n 65536 || true
ulimit -l 16384 || true
# Uncomment to configure a proxy for containerd
HTTP_PROXY=http://squid.internal:3128
HTTPS_PROXY=http://squid.internal:3128
NO_PROXY=10.0.0.0/8,127.0.0.0/16,192.168.0.0/16
"
```

[Hemanth Nakkina (hemanth-n)]

This feature has been implemented in sunbeam and available in 2023.2/edge