Move clear_volume back to it's own method

This just moves clear_volume back to it's own method
and also only calls volutils.clear if we actually
need to perform the clear operation.

Also takes out the hard-coded multiplier of 1024 and
uses units.GiB.  Maybe this should be a separate patch,
I'll spin another if there are objections.

Change-Id: I56f6690fbadc0d44eba48f49d29386d5b9dca195
Add method for unit tests to set logging level

Implements: blueprint cinder-testcase-set-loglevel
Change-Id: Id4bb7023659eda593cc5ac451aa580f68a1a6b9d
Merge "Move clear_volume back to it's own method"
Merge "Add method for unit tests to set logging level"