Use _set_instance_obj_error_state in compute manager set_admin_password

The set_admin_password operation is not a lifecycle operation like
building or deleting an instance, or resize for that matter, so replace
usage of _set_instance_error_state which does a host stats update with
_set_instance_obj_error_state which just sets the instance state to
error and handles any InstanceNotFound exception (in case we still want
to be a little paranoid).

Part of blueprint compute-manager-objects-juno

Change-Id: Ie1d1075095ed5d318d0d7a638526bc5cd40d4583
Merge "Use _set_instance_obj_error_state in compute manager set_admin_password"