(search-rpc) Lighttpd config: Adding a rewrite rule with a secret (specified in an included file) to allow updating the index directly from AppEngine.

Test Plan:
Manually copied config to /etc/lighttpd on production-rpc machine and restarted lighttpd.
Created a file /etc/lighttpd/khan-secret.conf and put in a random secret.
Verified that /solr/admin and /solr/update still give "Forbidden" errors, but /solr/update/SECRET does not.
Uploaded a sample command to /solr/update/SECRET/json and verified that it succeeded.

Reviewers: csilvers

Differential Revision: http://phabricator.khanacademy.org/D2257
Add a warning to install_solr.bash if secret is missing. (Phab review D2257)

Also remove initial update & cron job, and an indent fix.
Minor fixes from Phab review D2257.