[bgpvpn] Remove get_remote_client

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

There is a slight difference between tempest's
get_remote_client and the deleted one. See related
patch[2].

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
[2] https://opendev.org/openstack/tempest/commit/f52c759e5cc6da962ad63831c55dc140d05a2373

Change-Id: If2df2a0a3c6f8209a2e2d8d59a152bf3d685b0f1
2 files changed