Removes volumes redefinition in fake nova_client

Reasons:
- The redefinition of nova_client.volumes was making it,
  nova_client.volumes.volumes, if we needed to call the volume APIs
  from nova_client.
- nova_client.volumes is already defined when defining "class FakeClient".

Changes:
- Removes redefinition of nova_client.volumes.

Change-Id: I5981d461e5a0fd000214c35d5d0be58d86d6d368
Closes-Bug: #1292457
Merge "Removes volumes redefinition in fake nova_client"