Support remote servers of a different OS than server_master

I was trying to connect a CentOS server as a remote to a devshop server running ubuntu.

I was having more problems than expected, then realized web_group is set to www-data, not centos's "apache" user.

I tracked it down to Line 42 of provision/http/Provision/Service/http/public.php:

  function init_server() {
    parent::init_server();
    // System account
    if ($this->server->name == '@server_master') {
      $this->server->setProperty('web_group', _provision_default_web_group());
    }
    else {
      $this->server->web_group = d('@server_master')->web_group;
    }

It appears that servers are forced to use the web_group of server master.

Not sure yet how we might fix this, but it's something I see as a requirement: being able to support any server in a single aegir instance.

_provision_default_web_group() needs to be run on the server to get the right web group.