Reflect comments on bonding-in-ui PRs

1. NetworkManager refactoring (get rid of get_allowed_nic_networkgroups method).
2. Topology checker PoC is deleted also as this check is perfomed by other means.
3. Networking tests refactoring.
These fixes are related to comments on
https://review.openstack.org/#/c/70960/8,
https://review.openstack.org/#/c/73354.
They were postponed as non-critical.

Change-Id: If5c0dd7e703538706129a9da5a85182030b2de49
Merge "Reflect comments on bonding-in-ui PRs"