Store the node name as a local variable rather than recomputing it every time in different places

Summary: This ensures that we don't need to keep various computation in sync or ensure the data required to do the computation are available.

Test Plan:
Change the way the name is computed, in order toensure any test relying on the specific name would fail.

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

Reviewers: #bitcoin_abc, Fabien, jasonbcox

Reviewed By: #bitcoin_abc, Fabien, jasonbcox

Subscribers: Fabien, teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2247