add kerberos support

[arthurzenika]

Add kerberos authentification support.

It is likely we will contribute a patch for this in the near future.

[arthurzenika]

would it be ok to add a dependency on requests-keberos https://pypi.python.org/pypi/requests-kerberos ?

[whiteinge]

Kerberos support in Pepper would be fine as long as it is an optional requirement. In other words, no additional deps or steps must be required for Pepper to function as it does now. If Kerberos support is needed then additional software can be installed and configuration can be done.

Cross-linking saltstack/salt#24793 since the conversation there will also shape what Kerberos support in Pepper ends up looking like.

[arthurzenika]

OK. we'll find a way to have a configuration switch that enables this optional import.