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 f72e994971549dddfddef913703156b90b4335e6)
(cherry picked from commit 6ba3c139d06c1c5a15df805b1b74c987e1a974a9)
2 files changed