Instruct host to join a new pool

Parameters
name	type	description
session_id	session	valid session reference
master_address	string	The hostname of the master of the pool to join
master_username	string	The username of the master (for initial authentication)
master_password	string	The password for the master (for initial authentication)
Errors
name	arguments	description

• Message has both a synchronous and asynchronous version
• This is a custom message