This commit adds a new variable to lib/tempest to provide the plugins
that should be installed into common tox venv that gets created. In
order to make this work the workarounds to handle migrating to a common
tox venv have to be removed otherwise the plugins could be installed in
a venv that isn't used.
Change-Id: I63658b8d8dfa999e0feb79f8f2968f2b32e3ff57
Depends-On: Iab2e6e04b6c5795a4d0c8214564106525b942308
440464ccbb