Fix fake image_service import in v3 test_disk_config

In one place, we use the full module path for the image.fake module and
in another place we simply use 'fake', however we only import the
latter. This appears to only work because of the common.rpc import which
results in nova.tests.image.fake being defined. This import is going
away with oslo.messaging, though.

It looks like we use a mixture of approaches across the tests, so go
with the full path approach.

blueprint: oslo-messaging
Change-Id: Ida4c9e261a99d0610b557ba46c9f56927af21fef
Merge "Fix fake image_service import in v3 test_disk_config"