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

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

refactor the bad '+'-based string concatenations out.

Change-Id: Ic87634520a39b6ad05f7362a3c36e237e00b1840