bootstrap.sh: fix sites subdir when specifying options.uri

bootstrap.sh: fix sites subdir when specifying options.uri

Drush 9 tries too hard to get the sites subdir from the options.uri
setting.

This is wrong IMHO because options.uri is something relative to the
*web* root and can be subject to web server aliases and whatnot, while
sites-dir is something relative to the local filesystem; mixing the two
is counter-intuitive,

TO be on the safe side, pass the --sites-subdir option explicitly to
"drush site-install", to avoid a possible error when options.uri is
something valid but not expected by drush, like
http://localhost/~user/drupal/web

Fix the following error emitted in such cases:

 [error]  Could not determine target sites directory for site to install. Use --sites-subdir to specify.