state: stricter checking of container relations

Container scoped relations are now only allowed if there's exactly one
subordinate service involved. This prevents conditions that should
never have been allowed. The Uniter errors and exits when it
encounters relations without a subordinate service on one side.

This is part of the fix for LP #1382751.
state: InferEndpoints container relations tweak

InferEndpoints now ignores container relations unless there's exactly
one subordinate service involved.

This is part of the fix for LP #1382751.
Merge pull request #1075 from mjs/1382751-prevent-non-subordinate-container-scoped-relations

1382751 prevent non subordinate container scoped relations

This fixes LP #1382751.

Container scoped relations are now only allowed if there's exactly one subordinate service involved. This prevents conditions that should never have been allowed. The Uniter errors and exits when it encounters relations without a subordinate service on one side.

InferEndpoints now ignores container relations unless there's exactly one subordinate service involved.