Provide a way to configure Gadgets URL specifically

From http://confluence.atlassian.com/display/JIRAKB/Upgraded+to+JIRA+4.0+and+gadgets+do+not+display+correctly

Before displaying a dashboard, JIRA will make http requests to retrieve all the gadget specs for that dashboard. If the URL of the gadget spec is an absolute URL this could be a normal occurrence because the gadget spec is simply not being hosted any longer.

However if the URL is a relative URL as in the error above, JIRA is having issues retrieving gadget specs it's hosting itself. Eventually JIRA will no longer retrieve gadget specs via a HTTP request back to itself and make an API call instead. This has not been implemented as of this writing.

This is also the case if the JIRA instance is running on a virtual machine, which is running on an internal network. The issue is that the proxy is implemented by the hypervisor, doing the port forwarding
(and not apache). The hypervisor is not that easy to configure as an apache server.

It would be very handy if we could configure somewhere a parameter where the local gadget configuration files should be retrieved - instead of doing proxy server modifications.