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 #1235 from mjs/1396625-subs-with-container-scoped-relations-1.21

Allow container scoped relations between subordinate services - 1.21

This is the backport to 1.21 of #1234 .

(Review request: http://reviews.vapour.ws/r/545/)