[RFE] support multiple pools with targets in the same backend or different backends

[Dmitrii Shcherbakov]

With Designate v2 there is no need to create nameservers as with v1 and they are listed in pools.yaml which is loaded by designate.

It is possible to have multiple pools of the same or different type.

- name: pool-1
# ...
  targets:
    - type: bind9
# ...
- name: pool-2
  targets:
    - type: bind9
# ...
- name: pool-3
  targets:
    - type: infoblox
# ...
- name: pool-4
  targets:
    - type: powerdns

Docs:
https://docs.openstack.org/designate/queens/admin/multiple-pools.html
A fairly full example of pools.yaml:
https://github.com/openstack/designate/blob/master/etc/designate/pools.yaml.sample-multiple-pools