Removes retry of set_admin_password

* An RPC timeout may occur if an agent is missing and set_admin_password is
invoked. This causes 500 errors in the OpenStack API.
* Implemented a 501 error in API if the password set fails.
* Modified xenapi agent to use NotImplementedError instead of Exception.
* Fixes LP1061045

Change-Id: If7fab56c20f12e0490f4774e00004ed1d94242b9