[fwaas] Remove get_remote_client & check_vm_connectivity

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

Replaced methods:
    * get_remote_client
    * check_vm_connectivity

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: Iaf7f0ae7e2bd26ac98667fae3e86b86602c19f6f
1 file changed