Improved coverage for glance.api.*

Added new tests for glance.api.v2.image_data,
glance.api.v2.image_members, glance.api.v2.images,
glance.api.v2.schemas, glance.api.v2.image_tags,
glance.api.cached_images.

Part of blueprint improve-api-unit-tests

Change-Id: I2bd39e12366dd1b185bf60a8cd1d03a162ba5896
Merge "Improved coverage for glance.api.*"