Fix environment var prefix consistency across commands and sub commands

[timdrysdale]

The options for session host aren't used in production yet (defaults are good), so I've just noticed that the env var prefix is different in the help message for the session host command, and in the internal library vw that implements it. Neither is consistent!

in cmd/session/host.go change help message to say SESSION_HOST_* instead of RELAYHOST_
in internal/vw change environment var prefix from vw to session_host (lower case by convention of envconfig command, which then looks for a capitalised env var as we want)

Notes:

The vw.Stream() function checks the environment variables, so in this present implementation it needs to know the command which is calling it. The reasons for doing it this way were (a) there is only one function calling this library so the alignment of the prefix is possible, and (b) this approach theoretically lets you pick up changes in the environment variables, although I don't think I'm using that capability at the minute (changing where you host the instance seems problematic ....)

[timdrysdale]

Or indeed, just pass the prefix from the command to library, or pass the options (perhaps in a struct) so as to manage any hierarchy in the prefixes of the environment variables (although that would only occur for subcommands which does not apply here).

[timdrysdale]

Looking further afield there are inconsistencies about how sub-commands are formatting their environment var strings, e.g. BOOKUPLOAD_ instead of BOOK_UPLOAD, although this might be due to some limitation on having _ in the names. The more recent implementation of the session relay command, compared to the session host command uses viper to process the environment variables, and passes them as arguments to the function that does the heavy lifting. So we can look at increasing consistency here before going to v1.0 release.