Nutcracker: move declaration to role::mediawiki; parametrize

* Set owner / group / mode for config files.
* Only override nutcracker's default mbuf-size if caller requests it.
* Don't declare defaults. Especially if they conflict with nutcracker's!
* Don't assume a single server pool. Nutcracker supports arbitrarily many.
  We'll probably want to exploit this to ease the migration to a UNIX domain
  socket (if we choose to do that).
* Move declaration to role::mediawiki / role::beta.
* Fix Icinga alert to expect a single process.

Change-Id: Ifb4d25506ad5dddf046c3e6ba03f30541850d2be