passwd errors out as a attribute in [monitor] and [service] in maxscale.cnf

Maxscale errors out when using Maxscale 2.2 maxscale.cnf files that have passwd instead of password as attributes in monitor and services. It should allow both password and passwd in 2.3 and give a deprication warning if we're changing this. It is also in 2.3 documentation that passwd should still work.