better error handling for additional set relations in compute

set_relations() was attempting to relate rabbitmq-server to nova-compute.
There was no interface for that so an exception was being thrown. This
change fixes that exception and provides a more reliable way to relate
rabbitmq-server:amqp to nova-compute:amqp which is the proper requirement
for those services.

Signed-off-by: Adam Stokes <adam.stokes@ubuntu.com>
Feeling nitpicky today - why call this variable 'services' when juju.service() only ever returns one service?
I always have to think back and remember that when I read this