Add check for image_locations table

This commit adds a check for the image_locations table to the
_assert_tables method in TestGlanceManage. When migration 018
was added in change: I1536c9276bc807aac26849e37556b465e8b5c9eb
a new image locations table was created, but didn't update the
functional tests to check that it had been created.

Change-Id: If76702f623e201505c62d05e245003e3783d8574
Merge "Add check for image_locations table"
Merge remote-tracking branch 'm_src_repo/webob-issue' into build-temp