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>