Establishes a session with the VirtualCenter Management Server or ESX Server Web service using the token provided in a previously saved session file, or by using the user name and password provided via command-line, environment variables, or configuration file.
user_name: User account on the ESX Server or VirtualCenter Server host system.
password: Password for the user account.
filename: Full path and filename comprising the token saved from a previous successful connection. Use filename (instead of user_name and password) to re-establish session to the same server, or to establish new connection to a different server.
Returns a session object (token) that can be saved to a file and used in subsequent connections.