Open tcp/5666 in UFW

NRPE listens on tcp/5666 - without this port open, NRPE won't be able to work
correctly. This commit opens the port in UFW from any IP. Opening up this port
might seem like a bit of a security hole - and we can revisit it later if it is
deemed to be - but we will be locking down the hosts which NRPE can interact
with using the NRPE module.