[QENG-4075] Clean up pool manager, specs

Prior to this, `pool_manager.rb` allowed the `metrics` argument to be optional,
but at this point it will be an instance of `Vmpooler::Statsd`,
'Vmpooler::Graphite', or `Vmpooler::DummyStatsd`, so making this non-optional.

Cleaned up that file's tests, cosmetically, as well as recognizing that the
behavioral difference between graphite and statsd does not depend on the pool
manager.

lib/vmpooler/pool_manager.rb

@@ -1,6 +1,6 @@
 module Vmpooler
   class PoolManager
-    def initialize(config, logger, redis, metrics = nil)
+    def initialize(config, logger, redis, metrics)
       $config = config
 
       # Load logger library