Establishes a session with the VirtualCenter Management Server or ESX Server Web service using the provided credentials (user name, password) provided via command-line, environment variables, or configuration file.
service_url: The URL of the server to which the client connects.
user_name: User account on the ESX Server or VirtualCenter Server host system.
password: Password for the user account.
Session object that can be saved to a file for future use. See save_session for details.