Keep devstack's localrc for manila jobs

With this change all needed env vars for project can be changed
in pre_test_hook function stored in project's repo modifying localrc file.

Previous approach was in using 'source'd env vars, that
were lost with subprocesses.
Due to that all env vars, that are going to be used by devstack,
should be defined explicitly in parent process or via localrc.

Without this change both existing tempest jobs work with
singlebackend installation.

Change-Id: I41d1df4c467a68dbab35ad5771cffe02a466f6f7
Merge "Keep devstack's localrc for manila jobs"