Unify timing arguments

add small puppet duration parser and  support duration format for nagios settings

In order to avoid errors about time specficiations and unify the interface, 
add puppet support for duration parsing matching the Go support for http://godoc.org/time#ParseDuration

Use the duration function to unify the usage of `monitored_cron`.

@Bonko and @fungusakafungus please take a look.