Set a default HTTP_HOST for use with wp-cli

Our sunrise.php (and ms-settings.php if that's not around) require `HTTP_HOST` to be set in order to determine what domain and path are being requested.

If a wp-cli command is run without `--url` specified, `$_SERVER['HTTP_HOST']` is not available. This causes things to get confused on our end. Luckily, we can assume a default at the moment, wp.wsu.edu, and just specify it if the `WP_CLI_PHP_USED` server variable has been set.

See https://github.com/wp-cli/wp-cli/issues/730 for possible future changes.