Fix CLI installer when --dbname is not specified on command line.

Fix CLI installer when --dbname is not specified on command line.

When installing without an explicit `--dbname` argument on the command
line, the database is installed with the default value of $wgDBname
from DefaultSettings.php (which is `my_wiki`), but then the
LocalSettings.php file is written with the line:
    $wgDBname = "";
which overrides the default value and makes the wiki unable to
connect to the database created by the installer.

Adding `wgDBname` to the `$defaultVarNames` list fixes this problem.

Change-Id: I3792be9675aabe00b5497ed4203b12b89da8dc3a