Remove "s" from imange clients' method

The object is a single, but some methods of image namespace client
contain "s" which means multiple objects.
For the consistency of service clients, we need to remove the "s"
from these methods.

Partially implements blueprint consistent-service-method-names

Change-Id: Id21767e38d74e1275eea64f91143f06874a7b323
2 files changed