drbd: Add a DrbdOverview.get helper

This parses the output of drbd-overview so we can do things based on
this.
drbd: always run "drbdadm create-md" if the resource is unconfigured

We were relying on a notification from the template, but that's actually
wrong: there's nothing in the template that really matters there, I
think (except the disk, but if it changes, then we have other issues);
drbd: always run "drbdadm primary" if there's no primary role

There's no need for a notification: we have the information of whether
the role is missing or not.
drbd: use resource name for wait loop ruby_block

This helps avoid using the same name for multiple resources, which would
be an issue.