Allow submission of tenantName with initial auth

For non-rackspace installs, you need to supply tenantName with the initial authentication request in order to get a valid service catalog back.  This patch removes OpenStack::$template (switched to json_encode instead), and adds the ability to supply a tenantName during the initial auth process.

You would use this by doing the following:

    $connection = new OpenStack(AUTHURL,
	   array( 'username' => 'admin',
		   'password' => 'PASSWORD',
		   'tenantName' => 'admin' ));

This is somewhat related to #37, as the patch there won't really fix the problem fully (the auth request completes without warnings after that one, but nothing else will work as the service catalog is still empty)