chore: Rename MONGODB_TEST_LIVE to MARCONI_TEST_MONGODB

This patch renames the MONGODB_TEST_LIVE environment variable so that it
is consistent with MARCONI_TEST_SLOW. Testing for the renamed variable
was also DRY'd up using a decorator.

Change-Id: I6b08ae2ca2f86daf50df11219cc8c0a465e86c13