Provide more default clients for smoke tests.

 * Refactored DefaultClientManager to support default clients
   for image, identity and network in addition to compute.  This
   change is intended to faciliate the creation of
   cross-functional smoke tests.
 * In support of bug 1043980

Change-Id: I45c0e6bf5fde41936df8838b7434c0b2074f4a02
5 files changed