Enable using values from environment variables from tahoe.cfg

[amontero]

As I progress through dockerizing my setup, I've noticed that being able to use values from environment vars would make Tahoe-LAFS more easily configurable. It would be beneficial to use arbitrary env var values to replace any configuration option value from inside tahoe.cfg (perhaps also other files, to deal with secrets, too).

Also, Docker itself makes heavy use of env vars for host IPs and ports when linking containers.

Some kind of placeholder or token syntax instead of literal values, such as %VARNAME or {VARNAME} could be added to declare a value which would be env var replaced. The icing on the cake would be Bash syntax, which allows defaults ${VARNAME:-default_value}.

This would make Tahoe-LAFS a lot more dynamically configurable and more Docker-friendly, since env vars are commonplace.