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

Change-Id: Ibbe18b11d576881069a2171c6487a077a481f66d

add Image API(Glance) client and its mock tests(HTTPretty)

Change-Id: If78cff896599edaee72faf63d8753fd960910da4

add common types and nuked __init__.py in tests folder

Change-Id: Ie9d49f775f7a1c6e145fdbd0eaa4abc80230bf3f
add Visibility enum type for Image API 2.0 models

Change-Id: Ie258e09cb9ed4c7a50249cd911f503bb6b34dcaf