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>
ugh pep8

Signed-off-by: Adam Stokes <adam.stokes@ubuntu.com>
Change services to service variable

Signed-off-by: Adam Stokes <adam.stokes@ubuntu.com>
Merge pull request #58 from battlemidget/patch-fix-exception-for-nonexistent-charm

better error handling for additional set relations in compute