support wallet home directory

[ma2bd]

Motivation

Allow using --with-wallet NAME without setting 3 environment variables first!

Proposal

• Introduce a notion of home directory (e.g. set to a temporary directory for testing)
• Maintain backward compatibility

Test Plan

CI

Release Plan

This first PR is meant to be backward compatibly so that it can be backported on testnet_conway

[Twey]

Instead of a home directory could we have --with-wallet just take a directory path?

Otherwise we probably want a full suite of commands to do management — create, list, update, delete available wallets. But I'd much rather we just use a directory with a normal directory path, and the user can manage wallets with their normal filesystem tools.