Update disconnect_nodes API so it does take the node object directly

Summary: This ensures that we can update it later on to not depend on global state.

Test Plan:

./test/functional/test_runner.py --extended

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2216