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: I0911cda7fcf461b9786f9534689083c7ac3a2035
2 files changed