rpcauth: Make it possible to provide a custom password

Summary:

This adds the functionality to specify a custom password to
`rpcauth.py`, as well as makes the code (IMO) easier to understand.

Backport of core PR13146
https://github.com/bitcoin/bitcoin/pull/13146/files

Depends on D3759.

Test Plan:

make check

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D3780