add support for changing cinder perodic_interval

As discovered in tempest, when you create and delete volumes
quickly, and have a small volume storage pool, you can get cinder
scheduler to think you are "out of space" when you are not.

This is because cinder scheduler updates free space on it's
periodic job, which defaults to 60 seconds. We need control over
that value for the devstack gate otherwise we regularly get
overruns if we run too many volume tests quickly.

Work around for bug 1180976

Expect that this will get removed later if cinder gets a more
efficient way to update the scheduler for freed resources.

Also, don't be completely stupid about setting defaults....

Change-Id: I20e52e66fcc94b224476cdd14c88bd6981b4e617