Make role::eventlogging realm-aware

The server-side EventLogging setup is currently running on labs instance
'deployment-eventlogging', but it is not completely Puppetized. This patch
changes several hard-coded addresses to variables which are assigned a value on
the basis of realm.

Some acknowledged quirks:

- The value of $mongo_host is assigned on the basis of realm, but the values
  for production and labs are the same. This was done in anticipation of the
  MongoDB instance on vanadium eventually moving to a different server.
- When that happens, I'll also move the MongoDB class instantiation elsewhere.
- $::realm selectors do not specify a default case. I don't see the point in
  littering the repo with fail()s over 'realm' being set to something other
  than labs / production / fundraising. Custom errors are only needed when the
  error is relative to the idiosyncratic expectations / usage of the particular
  class, IMO.

Bug: 49723
Change-Id: Ifc11e5dc39333d70020150b86ba892575f0e39b1