env variable support for test DB url

env variable support for test DB url

So far DB url was retrieved from configuration file
If you want to use the CI chain (jenkins, testapi, test DB,
functest, ..) then you have a problem

calling functest will systematically point to the configuration
from the repo
Supporting env variable allow to avoid post processing in the
configuration file and as a consequence offer the possibility
to create a local DB to host local results

Change-Id: Ie64b8b265827d7f5724c7066a8c173de8cf012e2
Signed-off-by: Morgan Richomme <morgan.richomme@orange.com>