Allow baremetal-networks to be passed to devtest

Previously change I6e89dc98 allowed custom baremetal networks to be
configured in testenv.json.
This change allows you to pass in those custom entries as a json
file.

Change-Id: Iad5eb781fa236e71a40245d893f8f0b632b5c370
Merge "Allow baremetal-networks to be passed to devtest"