adtrustinstance: Properly handle uninstall of AD trust instance
    
    The uninstall method of the AD trust instance was not called upon
    at all in the ipa-server-install --uninstall phase.
    
    This patch makes sure that AD trust instance is unconfigured when
    the server is uninstalled.
    
    The following steps are undertaken:
      * Remove /var/run/samba/krb5cc_samba
      * Remove our keys from /etc/samba/samba.keytab using ipa-rmkeytab
      * Remove /var/lib/samba/*.tdb files
    
    Additionally, we make sure winbind service is stopped from within the
    stop() method.
    
    Part of: https://fedorahosted.org/freeipa/ticket/3479