Separate capabilities service method from account_client

list_extension() method in account_client is to lists
the activated capabilities of swift.

That is separate API than account service and should have
separate service client.
- http://developer.openstack.org/api-ref/object-storage/?expanded=list-activated-capabilities-detail
Partially implements blueprint consistent-service-method-names

Change-Id: I344c424020f9e10832040935fd0df43e596003c3
9 files changed