Configured lighttpd to reverse proxy CloudSearch.

Summary:
I also updated the Vagrantfile to make testing easy.

Unfortunately the best way I could come up with to do this was creating
a CGI script. See http://phabricator.khanacademy.org/T2427 for reasoning
and more information.

Test Plan:
- Tested that Vagrant can bring up a machine reliably reverse proxying
  CloudSearch AND serving Solr.
- Tested that URLs that weren't whitelisted receive 403 error.
- Tested that provisioning script is idempotent by repeatedly applying
  it.
- Ensured that both publishing with Solr and publishing with
  CloudSearch works correctly.
- Ensured that queries to both services work correctly through /search
  (on the actual site).

Reviewers: benkomalo, csilvers

Reviewed By: csilvers

Maniphest Tasks: T2427

Differential Revision: http://phabricator.khanacademy.org/D8379