Clean up create_disk() a little
Clean up create_disk() a little

The create_disk() helper had some redundant checks and dead code. This
refactors it to put all the stale cleanup at the top, and groups the
new actions together with more relevant comments to make it easier
to understand.

Change-Id: I1f6218a1994e66786ed9a8065e30bcceec7b8956

nawawi/openstack,openstack-dev/devstack,openstack-dev/devstack,nawawi/openstack