monitoring: move decom_host define into autoload

ERROR: decommission_monitor_host not in autoload module layout

make the file name match the class name to fix that

and minor warnings:
 double quoted string containing no variables
 ensure found on line but it's not the first attribute 

Change-Id: I3b852672e910d1efa2f920eb8ed556579d70530c
Merge "monitoring: move decom_host define into autoload" into production