Fix instance_exists uuid
Fix instance_exists uuid

When the uuid cache was removed it broke the instance_exists()
method because the instance uuid was no longer translated to
the PowerVM uuid value. The fix is to add back in that
translation that use to be done by the cache lookup().

Change-Id: I60c838a09f5cdd5f6b8212250ad3874403e6711d

stackforge/nova-powervm,openstack/nova-powervm,stackforge/nova-powervm,openstack/nova-powervm