late check pysnmp module to not log error on startup

This help not show bellow error on startup:

````
Apr 26 03:22:26 backup diamond[22844] ERROR diamond snmp.__init__:52 pysnmp.entity.rfc3413.oneliner.cmdgen failed to load
Apr 26 03:22:26 backup diamond[22844] message repeated 2 times: [ ERROR diamond snmp.__init__:52 pysnmp.entity.rfc3413.oneliner.cmdgen failed to load]
````

I'm not a snmp collector user, so I'm not testing whether those collectors still work well after my change.