[fwaas] Remove _create_subnet & setup_clients

As tempest.scenario.manager was announced stable interface in Tempest
27.0.0[1] it can be now reused in plugins.

Replaced methods:
    * _create_subnet
    * setup_clients

Etherpad concerning this effort:
https://etherpad.opendev.org/p/tempest-scenario-manager-cleanup

[1] https://docs.openstack.org/releasenotes/tempest/v27.0.0.html#release-notes-27-0-0

Change-Id: I7ee881e8031a2198f3b307667bfd3e676459758d
2 files changed