Upgrade tests' mocks to match glance_store

Latest glanec_store release added a new `context` keyword to some
functions. This broke backwards compatibility for some of the tests in
master and juno.

This change adds the corresponding keyword to the mock calls.

Closes-bug: #1391437

Change-Id: I5e58462e4c076b3ea56e27361eb6c3b0aafa2493