Provision on different drivers dynamically

Provision on different drivers dynamically

We want TOX_POSTGRESQL and similar to be the fixed variable
that is configured from CI environment.   These variables should refer
to database servers but individual drivers like asyncpg mysqlconnector
etc. should come from local tox.ini.  add a new system to generate
per-driver URLs from a simple list of hostname-based URLs delivered
from CI environment.

Change-Id: I4267b4a70742765388c7e7c4432c1da9d9adece2