Support Kerberos authentication
    
    This patch applies to deployment with Apache and mod_wsgi.
    
    Kerberos authentication is handled by Apache module mod_auth_gssapi,
    both GSSAPI negotiation and fallback Basic authentication are
    supported.
    
    After succeeding to authenticate a user, then ODCS will get user's
    Kerberos principal and be able to retrieve groups from LDAP server for
    subsequent authorization based on defined groups.
    
    Major changes:
    
    * mod_auth_gssapi configuration is added, that could be put into proper
      place along with mod_wsgi configuration is for running ODCS.
    * Add User and Group models to store user information in database.
    * Add configs for query from LDAP server
    * Multiple authentication backend is supported.
    
    Signed-off-by: Chenxiong Qi <cqi@redhat.com>