Do not set wgServer in cli install unless explicitly passed

Do not set wgServer in cli install unless explicitly passed

Currently, if we do not pass a wgServer value (via --server),
the CLI installer sets the value to the default value
specified in DefaultSettings.php - which is to 'guess' by
calling into WebRequest::detectServer. This yields terrible
results, since WebRequest::detectServer expects to be working
in the context of a Web Request - and hence with HTTP Host
header information. Since calling from the CLI does not give
it host header information, it falls back to 'localhost',
which is not the value you usually want.

If we just do not set wgServer when it is not specified,
it is automatically calculated on every request by
WebRequest::detectServer, which does a splendid job.

Bug: 55376
Change-Id: I5436dd8c340604cbb59406a507188e11c8f86e86