Fix of using concurrency for tempest in manila job

tox's venv takes only one argument, 'concurrency' was second
and wasn't consider, so tests were running in 8 parallel threads,
because 8 processors were available.
It caused fail of whole test suites due to absense of resources
for creation of volumes.
Changed concurrency to 2 (two), as guaranteed variant without jumping out
from resource pool and changed size of volume groups to make it possible
create more volumes.

Change-Id: Ia870373687988aec702c97f95047d4ac8903bb66
Merge "Fix of using concurrency for tempest in manila job"