Rework entities

[pehala]

Overview

• Extracted basic CRUD logic from entities into common superclass KeycloakAdminEntity.
  • Entity parameters are available directly (e.g entity.id, entity.serviceAccountsEnabled).
  • Entity is fetched lazily when needed.
  • Update request shouldn't change unique identifiers, that will make the object not work anymore.
  • Update method doesn't transform snail_case into camelCase.
• Added more methods to ClientRoles, Realms and Clients collections.
• Added Token class which represents refreshable Token.
  • Token is now returned by most method in KeycloakOpenIDConnect class.
  • Token is automatically refreshed when set up as authentication header.
    • NOTE: I am not sure about validity of refreshing code for authentication by client_id and client_secret
• Added Pipfile for easier setup
• Edit test to reflect these changes.
• Removed some called_only_once checks because they didn't make sense anymore.

NOTE: I noticed that you support python 2.7 (at least it seemed from the code), I am not sure if that would still work after this PR.

[Peter-Slump]

Hi @pehala thanks for the changes! I see where you are going and I think it's a nice addition. However since there are breaking changes I decided to not pull it into master but use is as the base for a new major version. So I merged your work into the develop branch.