[Docs] Authentication and authorisation

[k-wall]

The documentation is currently missing content talking about authentication and authorisation. This should be added.

We should have an general high-level description of how authentication and authorisation work in EnMasse so that readers understand the security model. Perhaps this could form part of a Security chapter (which would also include information about TLS and SASL).

In addition, it should be clear the differences that arise on different platforms. For instance on OpenShift, by default, the user is given the option to authenticate using an OpenShift credential (#1533 #1534)

[lulf]

1.0 is a good opportunity to add this.