[cleanup] De-globalize rpc configuration variables

Summary: As per title

Test Plan:

make check && ./test/functional/test_runner.py httpbasics

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: jasonbcox, teamcity

Differential Revision: https://reviews.bitcoinabc.org/D2017