…present without a standalone puppetdb replica.

To try and avoid dealing with the complexity of PuppetDB database host
picking at the end of the manifest, see if having a sane class parameter
default solves the problem cleanly. If it does, we can do a simple
switch on replica/no-replica to create the B groups.

It doesn't like having an optional parameter come before a non-optional
one. Since parameters are moving around a bit, add some comments to
clarify their intended usage.