Specify how frequently dashboard checks for data update

New config parameter 'dashboard-update-interval' is introduced.
The interval specifies how frequently dashboard should check
for updates in seconds. Default value is 1 hour (3600 seconds).

This allows to avoid situation when dashboard needs to update data
(rebuild indexes, purge cache) on every request that happens when
processor runs.

Closes bug 1324653

Change-Id: Ie86568eb5ec758e4f93bc3583f7bcce0f19df72f