In some environments zuul operators may have to rely on external
database providers. In this case it can be cumbersome to get extra
databases for each test environment. Adding an optional prefix to the
table names makes it possible to gracefully run several zuul
deployments against the same database and ensure they're still
isolated against each other.
Change-Id: Ib9948d6d74f4dc2453738f5d441e233e39e7f944