diamond::collector: make settings default to undef

Fixes:

Error: Could not retrieve catalog from remote server: Error 400 on SERVER: Must
pass settings to Diamond::Collector[Network] at
/etc/puppet/modules/diamond/manifests/init.pp:95 on node argon.wikimedia.org

Change-Id: I0251cf772c58248066d82aa9e3d02a159d71d52a