Refactor TestNode arguments management

Summary:
Refactor only, no functional change:

• rename self.args to self.default_args
• separate the binary name from the arguments list
• add facilities to clear/add/remove default arguments
• update test_framework and feature_config_args to use these facilities

Test Plan:

./test/functional/test_runner.py

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: jasonbcox, teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2535