Removes accounts mgmt extension use of non-existent Nova extension

The accounts mgmt extension uses a Nova accounts extension that doesn't
exist. This removes the use of Nova accounts and instead lists all
instance ids for an account GET call. Account list still returns the
number of instances for each account.

Change-Id: I8a4387aec2e124dfbfb5c55a62fe4154e5e38544
Closes-Bug: 1130924