moved extra_config merge to prevent overwrites

if graphite is enabled, whatever the extra 'graphite' config is set, it's being overwritten by line 76.

config_hash[:graphite] = { :legacyNamespace => node[:statsd][:legacyNamespace] }

this change merges extra_config right at the end, to fix the issue.
Merge pull request #21 from Igorshp/master

moved extra_config merge to prevent overwrites