Add a local variable for the rpc port

Summary: This ensure that we can get rid recomputing the port number based on state that comes from outside the TestNode instance.

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/D2250