provision/docker: always copy the database connection in tests
db: introduce the type Collection

This will allow us to better manage connections.
db: use Collection instead of mgo.Collection

I will still check the impact of this change, but it should not be
something bad.
router: use db.Collection instead of mgo.Collection
provision/docker: make sure tests always close connection copies
provision/juju: use db.Collection instead of mgo.Collection