[QENG-4075] Unify graphite and statsd for the pool manager

Prior to this, the `pool_manager.rb` library could take handles for both
graphite and statsd endpoints (which were considered mutually exclusive),
and then would use one. There was a bevy of conditional logic around sending
metrics to the graphite/statsd handles (and actually at least one bug of
omission).

Here we refactor more, building on earlier work:

 - Our graphite class comes into line with the API of our Statsd and DummyStatsd classes
 - In `pool_manager.rb` we now accept a single "metrics" handle, and we drop all the conditional logic around statsd vs. graphite
 - We move the inconsistent error handling out of the calling classes and into our metrics classes, actually logging to `$stderr` when we can't publish metrics
 - We unify the setup code to use `config` to determine whether statsd, graphite, or a dummy metrics handle should be used, and make that happen.
 - Cleaned up some tests. We could probably stand to do a bit more work in this area.

lib/vmpooler.rb

@@ -54,10 +54,6 @@ module Vmpooler
       end
     end
 
-    if parsed_config[:graphite]['server']
-      parsed_config[:graphite]['prefix'] ||= 'vmpooler'
-    end
-
     if parsed_config[:tagfilter]
       parsed_config[:tagfilter].keys.each do |tag|
         parsed_config[:tagfilter][tag] = Regexp.new(parsed_config[:tagfilter][tag])
@@ -65,7 +61,6 @@ module Vmpooler
     end
 
     parsed_config[:uptime] = Time.now
-
     parsed_config
   end
 
@@ -77,19 +72,16 @@ module Vmpooler
     Vmpooler::Logger.new logfile
   end
 
-  def self.new_graphite(server)
-    if server.nil? or server.empty? or server.length == 0
-      nil
+  def self.new_metrics(params)
+    if config[:statsd]
+      Vmpooler::Statsd.new(config[:statsd])
+    elsif config[:graphite]
+      Vmpooler::Graphite.new(config[:graphite])
     else
-      Vmpooler::Graphite.new server
+      Vmpooler::DummyStatsd.new
     end
   end
 
-  def self.new_statsd(params)
-    return DummyStatsd.new unless params
-    Statsd.new params
-  end
-
   def self.pools(conf)
     conf[:pools]
   end