diamond: default collectors to enabled = true

Having the collector config set any value other than 'true' for 'enabled'
doesn't make sense (when we disable the collector in Puppet, the config file is
simply removed). The docs for diamond/manifests/init.pp already indicate that
enabled is set to true by default, but I must have omitted it from my patches
for some reason.

Change-Id: I8e574c3620323a75454bfe77d8e40f8570d4e54e
Merge "diamond: default collectors to enabled = true" into production