ipatests: added unlock_principal_password and change_principal
    
    The unlock_principal_password unlocks the (new) user by running
    ldappasswd as the user.
    
    change_principal is an context manager that changes identity
    for the supplied api object by disconnecting and reconnecting
    the rpcclient in and outside of requested kerberos context.
    This context manager allows to run tests that cannot be
    executed as an admin user which can for example override
    an CA ACL.
    
    https://fedorahosted.org/freeipa/ticket/57
    
    Reviewed-By: Martin Basti <mbasti@redhat.com>