[tests] only use 2 nodes in interface_rest.py

Summary:
Partial backport of core PR12766 (commit ade5964)
https://github.com/bitcoin/bitcoin/pull/12766/commits/ade5964e3f8c5039878e1296311bb0337c152d2f

Depends on D3768

Test Plan:

./test/functional/test_runner.py interface_rest

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D3769