peer and subordinate relation types

[PietroPasotti]

At the moment it is possible to model peer and subordinate relations using state.Relation, but it's not quite wieldy.
We should consider adding PeerRelation and SubordinateRelation types that offer more sensible defaults or hide certain arguments that don't make sense in that context.

PeerRelation:

• remote_app_name should be the same application name as the unit being tested
• remote_app_data should point to local_app_data or disappear
• in general naming of 'remote' is dubious in a peer-relation context

SubordinateRelation:

• instead of 'remote', should we be talking of 'primary'?
• there is always exactly one 'remote unit'.

mix cleanup:

• remove Relation.leader_id

[PietroPasotti]

closed in #19