Add api validation schema for image_metadata

Add schema file to do api input validation for image_metadata api.
And some unit test case fix for the 'update' action
- create accepts 'metadata' instead of 'meta'
- update accepts 'meta' instead of 'metadata'
- update_all accepts 'metadata' instead of 'meta'

Partially implements blueprint v2-on-v3-api
Change-Id: I364860d5514d94a146b56c74ff3f3950a3b8d5b3
Merge "Add api validation schema for image_metadata"