contint: do not cache api/json calls

The Jenkins server is being the misc varnish, when creating a Jenkins
job the script we are using first verify whether it exists (which cause
a 404), then create the job and verify it got created.  That last step
is being served a 404 because of the cache.  That cause us to create
each jobs individually instead of in bulks.

More details:
http://lists.wikimedia.org/pipermail/wikitech-l/2014-February/074829.html

This patch set the header "Cache-Control: no-cache,must-revalidated" for
any URL matching api/json.

Live hacked on the production machine gallium and confirmed the jobs can
now be created properly.

Change-Id: I3391de4fffcbf77624ad566ef541dbd8621faa76
Merge "contint: do not cache api/json calls" into production