Allows slave to use authenticated Git/HTTP protocol

When using Gerrit as slave, it is possible to enable
Git/HTTP protocol but only without authentication,
making it quite ineffective for a production usage
other than anonymous browsing.

With this change the HTTP Authentication is available for
slaves as well, making it a viable option for registered
users.

Change-Id: I1805862154c776fa6c44571efa13074fb04bbe9d