(POOLER-71) Add dummy authentication provider

Previously it was difficult to do local development as VMPooler requires an LDAP
service for authentication. This commit adds a dummy authentication provider.
The provider has passes authentication if the username and password are
different, and fails if the username and password are the same.  This commit
also updates the documentation in the config YML file.

lib/vmpooler/api/helpers.rb

@@ -56,6 +56,8 @@ module Vmpooler
 
       def authenticate(auth, username_str, password_str)
         case auth['provider']
+          when 'dummy'
+            return (username_str != password_str)
           when 'ldap'
             require 'rubygems'
             require 'net/ldap'