Read in consensus defaults.

Summary:
Now that we have a get_srcdir() utility function,
we can determine the location of consensus.h more reliably.

Avoids duplication of these default definitions.

Test Plan: ../qa/pull-tester/rpc-tests.py -extended

Reviewers: #bitcoin_abc, deadalnix, sickpig, kyuupichan, awemany

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D305