increase fastcgi read timeout in nginx template

so that it doesn't close connection for long running tasks like
PDF generation

Change-Id: I06aa0f13762a88902927ab88d456f03d431d143d
Reviewed-on: http://review.couchbase.org/27484
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Pavel Paulau <pavel.paulau@gmail.com>