state: relaxed checking of container relations

Container scoped relations are only allowed as long a there's at least
one subordinate service involved. Previously there had to be exactly
one subordinate service but it turns out some people where relying on
being to establish container scoped relations between 2 subordinate
services.

This is part of the fix for LP #1396625.
state: relaxed InferEndpoints container relations check

InferEndpoints now allows container relations as long as there's at
least one subordinate service involved.

This is part of the fix for LP #1396625.
Merge pull request #1234 from mjs/1396625-subs-with-container-scoped-relations

Allow container scoped relations between subordinate services

This fixes LP 1396625 by relaxing the checks for creation of container scoped relations. One commit changes AddRelation and the other changes InferEndpoints.


(Review request: http://reviews.vapour.ws/r/544/)