Change-Id: Ie23604c34ed9fb17eeb15bf73db9019ca9ebc7d8

Previously, all subnet properties were updated if any value did not match.
But this behaviour caused errors like e.g. "Current gateway ip 192.168.0.1
already in use by port [ID]. Unable to update." even if that gateway was
not about to be changed.

Task: 40927
Story: 2008172
Change-Id: I049b0dade4c7ea3e1ef24777ae558f650caa136c

Fix subnet idempotency for allocation pools, see the linked story.
Return updated subnet information.
Remove adding allocation pools that were introduced in
Ib8becf5e958f1bc8e5c9fd76f1722536bf1c9f1a
in order to add allocation pools, either add new variable or
recreate the subnet.

Task: 41307
Story: 2008384

Change-Id: Ibe808227de159c6975dc94ef8ad0ab03a9345e17

This is separate from the previous patch - it's just the results
of running the script so we can review the two a little independently.
We should probably squash them.

Change-Id: I838f15cf4a32455a5be20033c8ddc27db6ca15c0

As per the REST Networking API v2.0 (Subnet) docs, if subnets pools
are not specified, OpenStack networking automatically allocates pools
covering all IP addresses in the CIDR.

In custom vendor specific environments, subnets can be created without
allocation pools via UI due to which subnets will not have any existing
allocation pools. Under this scenario, module throws an
"IndexError: list index out of range" error.

Also, allow to add more allocation pools on top of existing allocation
pools.

Change-Id: Ib8becf5e958f1bc8e5c9fd76f1722536bf1c9f1a