moved extra_config merge to prevent overwrites

An issue was introduced in https://github.com/librato/statsd-cookbook/pull/20:

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 moves the merge to after all config_hash mutations are done, giving extra_config parameter full control back again.