Don't throw ValueError for invalid volume id

If a user runs something like "cinder-manage volume delete a1234",
a ValueError is thrown because it fails to cast to int.
Catch this and treat the parameter as an id which will result in a
later VolumeNotFound error rather than breaking this way.

Change-Id: I95a9b9d7628cebe4b6d855ea925b0ad3a5f1c4c4
Merge "Don't throw ValueError for invalid volume id"