core: list VAGRANT_* env vars in debug log

Help support and debugging by printing all `VAGRNAT_*` environment
variables that have been set by either user or Vagrant.

lib/vagrant.rb

@@ -60,6 +60,9 @@ global_logger = Log4r::Logger.new("vagrant::global")
 global_logger.info("Vagrant version: #{Vagrant::VERSION}")
 global_logger.info("Ruby version: #{RUBY_VERSION}")
 global_logger.info("RubyGems version: #{Gem::VERSION}")
+ENV.each do |k, v|
+  global_logger.info("#{k}=#{v.inspect}") if k =~ /^VAGRANT_/
+end
 
 # We need these components always so instead of an autoload we
 # just require them explicitly here.