monitoring: do not warn for > 1 salt-minion procs

When we run salt commands, salt-minion forks and executes. This raises
icinga warning alerts for > 1 processes. Silence them by requiring
minimum 1, but keep the critical threshold of 4 for now.

Change-Id: I9ba1e4e9c39488121982f5f2f029998d7a8cb349