Remove accounts 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 shows all
trove instance ids associated with the account. Accounts index is unchanged
and returns the total number of instances per account for all accounts.

Change-Id: I8a4387aec2e124dfbfb5c55a62fe4154e5e38544
Closes-Bug: 1130924
Merge "Remove accounts use of non-existent Nova extension"