Fix minor non-important error in cache_devstack

Found this while porting to diskimage-builder - the basedir value
is passed but not used. It's ok, because we happen to be using the
same value- but it's an error waiting to happen.

Change-Id: Ic2f39a9ff54c7bcdf3fcd48c4c6b2f98f6a88cf0
Merge "Fix minor non-important error in cache_devstack"