Add support for configuring PEAR

It seems that PHP's extension manager PEAR doesn't support `http_proxy` env var, but has its own [configuration command](http://pear.php.net/manual/en/guide.users.commandline.config.php), which should be used for [setting proxy configuration](http://stackoverflow.com/questions/2266491/how-to-use-pear-behind-proxy).

Again we need to wait for mitchellh/vagrant#2405 so we can configure pear after it is installed in a provisioner run.