add Image API v1.0(Glance) models and their unit tests(pytest)

- cherry picked from supersceded branch
- add __init__.py to enable test namespaces
- move tests/ to metatests/
- fix string formatting consistency
- refactor the bad '+'-based string concatenations out.
- add Image API(Glance) client and its mock tests(HTTPretty)
- add common types and nuked __init__.py in tests folder
- add Visibility enum type for Image API 2.0 models
- add Images API v2.0 model
- fix pep8 violations
- add previously missing file

Change-Id: Ic87634520a39b6ad05f7362a3c36e237e00b1840
Merge "add Image API v1.0(Glance) models and their unit tests(pytest)"