Allow implicit juju-* relations to be valid.
- https://juju.ubuntu.com/docs/authors-implicit-relations.html
- juju-* is a namespace for implicit juju relations all charms support. This
adds a check for relations that start with juju- and auto approve them in
proof.
- Charms are not allowed to create any relation of their own that start with
juju-, so it's safe to assume anything that starts with this is a safe
implicit relation.
https://codereview.appspot.com/23660043/
R=benji.
Approved by Juju Gui Bot.