setting to completely override server hostname

setting to completely override server hostname

Labs has instances hostnames forged in sequences which make them not
that much useful for human readable output. Labs does publish a
meaningful instance name as an env variable though, so we could use it
to get better output.

This patch introduce $wgOverrideHostname (default false) which when
set will override the hostname returned by wfHostname().

Change-Id: Ibdba007cc4025fa0b0ebef39b5669c32852a95a7