session_server options do not support environment variables

Description

Currently, many of the options in common/config.go support getting their values from the toml configuration file, or environment variables. It does not look like any of the options in the [session_server] section support getting values from environment variables. This makes dynamic deployments more difficult, as the toml config file needs to be manually updated with sed instead of having a different env var given to the process at start time.

Proposal

Allow environment variables to provide values for the ListenAddress, AdvertiseAddress, and SessionTimeout values.