Changes to cast variable as integer as XML API fails.

Code change to ensure that the type of the size variable is correct
when cinder is invoked through XML API. The extend volume code path
differs from volume create where the flow portion verifies that the
size variable is an integer; for extend the earliest place to cast
the size variable is in volume_actions.py.

Change-Id: Iaaed463f988769b8093604cd3b677b270dd2c5cd
Closes-bug: 1277233
Merge "Changes to cast variable as integer as XML API fails."