(POOLER-72) Add Dummy Provider

Previously the only VM Provider was vSphere however this made testing and making
changes difficult as it required a functioning vSphere instance.  This commit
adds a Dummy Provider which presents a VM provider to Pool Manager but manages
provisioned "VM"s in a hashtable.  The Dummy Provider can also be configured to
randomly fail operations and take random amounts of time to perform operations,
 such as cloning a VM, which is useful to see how the Pool Manager copes with
these events.

This commit also updates the configuration YAML documentation and adds
appropriate unit tests.

lib/vmpooler/providers.rb

@@ -1,4 +1,4 @@
-%w(base vsphere).each do |lib|
+%w(base dummy vsphere).each do |lib|
   begin
     require "vmpooler/providers/#{lib}"
   rescue LoadError