juju-lint should support relations that if present are considered an error

[Tom Haddon]

Per https://bugs.launchpad.net/charm-nova-compute/+bug/1802910 we had a situation recently where the presence of a relation between cinder and ceph-mon was a problem. It would be nice if we could specify in juju-lint that the presence of certain relations should be considered an error.

[Andrea Ieri (aieri)]

Another example: filebeat <-> elasticsearch when filebeat <-> graylog <-> elasticsearch is already present.

[James Hebden (ec0)]

Per 1805701 we're not currently checking relations in the linting code, so this bug should be considered whilst implementing relation checks - i.e. ensuring a relation is absent between two charms.