[qa] Change connect_nodes_bi API to not assume an array of nodes.

Summary: As per title.

Test Plan:

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

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2217