Add support for multiple realms to authenticators

[dene14]

Right now we do support different realms as long as they use different authenticators. However its not possible to manage more than 1 realm within the same authenticator (for e.g. multiple Github organizations).

couple of possible scenarios:

1. Company has acquired another company - and due to historical reasons users distributed between two Organizations on github
2. Company owner has service agreement contract with another company that provides development/services for the company, so we need to be able to provide access to k8s from both orgs

Most probably should be done in conjunction with #206