container.slave defaults to --slave behavior

When running Gerrit in large environments it's sometimes necessary
to configure Gerrit slave installations.  This change adds the
configuration option container.slave, if set to true gerrit.sh
is called with the slave switch. If no value is set (or any other
value), Gerrit defaults to master mode.

Change-Id: Ic4103e0d3d952e0d56b6aa5101d20c960588772a