Make Tempest repo stable for Manila

Manila has Tempest plugin for mainstream Tempest.
Now, our jobs run over latest Tempest.
It causes incompatibilities from time to time.
Fix it by checking out Tempest repo HEAD to some specific commit that is
compatible with Manila plugin.

Implements blueprint stable-base-for-functional-tests

Change-Id: I5760461c1ebf8cf53ccfae55606fe6036c915e02
Merge "Make Tempest repo stable for Manila"