[buildbot] Extract the build configuration retrieval and parsing to its own function

Summary: This will be called from another place in another diff. There is no change in behavior.

Test Plan:

./contrib/teamcity/build-configurations.py check-buildbot

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Subscribers: PiRK

Differential Revision: https://reviews.bitcoinabc.org/D13661