As added in notes for INSTALL_TEMPEST variable we need to set
this as False for stable branch so that devstack does not
install Tempest at system wide.
- aa2821eb89/lib/tempest (L61)
This should be done at the time when we cut the stable branch but
we forgot to do that for ussuri and victoria.
Change-Id: I23c77f98c2e969d8046d5212b883e343c36cd1b1
(cherry picked from commit 9a8c062312)