(maint) Output logging to STDOUT when debugging

Previously log messages would only go to the log file.  This commit adds
additional functionality to the logger by output to STDOUT if the environment
variable VMPOOLER_DEBUG is set.

lib/vmpooler/logger.rb

@@ -11,9 +11,11 @@ module Vmpooler
     def log(_level, string)
       time = Time.new
       stamp = time.strftime('%Y-%m-%d %H:%M:%S')
-
       open(@file, 'a') do |f|
         f.puts "[#{stamp}] #{string}"
+        if ENV['VMPOOLER_DEBUG']
+          puts "[#{stamp}] #{string}"
+        end        
       end
     end
   end