nrpe: adjust Zuul process monitoring

With the new Zuul version, it forks a process to run the Gearman server.
The nrpe checks needs to be adjusted to ensure we have exactly TWO pids
running.

Change-Id: I6a05c1b5faa0f9f732f9b6d99a44d0076e35e857
Merge "nrpe: adjust Zuul process monitoring" into production