Reduce mongo log spam by tuning what roles are used when logging in to a...

Reduce mongo log spam by tuning what roles are used when logging in to a database.

 syslog is filled with:
warning: No such role, "readWriteAnyDatabase", in database juju. No privileges will be acquired from this role
warning: No such role, "clusterAdmin", in database juju. No privileges will be acquired from this role
warning: No such role, "readWriteAnyDatabase", in database presence. No privileges will be acquired from this role
warning: No such role, "clusterAdmin", in database presence. No privileges will be acquired from this role

So the roles used to log into the juju and presence databases are reduced.

Partial fix for: https://bugs.launchpad.net/juju-core/+bug/1365828

NOT LGTM.

The Machine Agent is the one responsible for setting up the ReplicaSet data when you use "juju ensure-availability" *not* the "admin" user.

So we need Machine agents to have "clusterAdmin" rights.