Do not test subnetpools in TF case

TF do not support subnet_allocation extension.
Also bring new method, that can be used as common in
tests to check TF backend.

Related-Prod: https://mirantis.jira.com/browse/PRODX-23390
Change-Id: I369de0530e940c3ddd35541f175f420e576063c5
(cherry picked from commit 14202aa048de2b6e504c8dd1762400c5bf19d7d2)
2 files changed