Make USE_MARIADB overrideable
Make USE_MARIADB overrideable

The $USE_MARIADB environment variable needs to be overrideable otherwise
settings in tripleorc/devtestrc are not going to have any effect.

Change-Id: I3e4bb4005068b2b5143b2f5d7eecd6ef1b4406ad

rdo-management/tripleo,rdo-management/tripleo