kerberos client plugin

Enable kerberos when doing authentication.
It will reuse any existing kerberos token and use it if the server
uses WWW-Authenticate: Negotiate.
depends on to requests-kerberos

Co-Authored by: Adam Young <ayoung@redhat.com>
Co-Authored by: Jamie Lennox <jamielennox@redhat.com>

bp: kerberos-auth-plugin

Change-Id: Idf02bf27b5933c00827dd08d11ac131896184ad8