Rework RHSSO into a more general OIDCProvider interface

[pehala]

• Introduce OIDCProvider interface
  • This enables us to quickly swap between OIDCProviders (planned Auth0 and RHSSO for now) without too much of a hassle
  • Add Token class which wraps the Token and can refresh itself if needed
  • Add oidc_provider fixture, which should replace rhsso_service_info almost everywhere
• Cleanup of RHSSO interface
  • Remove unneeded methods
  • Remove RHSSOAdmin class
  • Move the initialization from fixture to RHSSOServiceConfiguration class
  • Move it into the new oidc module

Fix #68