In $wgConf, allow wgCanonicalServer to be false or absent

According to the doc comment, $wgCanonicalServer can be false, and this
is equivalent to it being set to $wgServer. So don't require it to be
set for foreign wikis in $wgConf, just use the same interpretation.

This makes it slightly easier to set up CentralAuth test servers.

Change-Id: I5eb11669236ed2a71d08f53a1393a3d5e8f5f770