NIFI-4499 Updated default content-viewer URL property to be relative …

[jtstorck]

…rather than assuming it is deployed at the root context.

To test this PR:

• Build NiFi and start it with the default nifi.properties file.
• Configure a proxy, and point it at NiFi.
• Access NiFi via the proxy
• Add a processor that creates flowfiles with viewable (ie, text) data, such as GenerateFlowFile.
• Add any other processor, such as UpdateAttribute.
• Route the first processor to the second processor
• Start the first processor to get some data into the outgoing queue, then stop the flow.
• Right-click on the queue and view the queue listing
• Select the info icon next to one of the queue items
• From the Details tab, click View. A new tab should open with the content viewer and the flowfile data can be viewed. Close the content-viewer tab. Close the dialog with the FlowFile details.
• From the queue listing dialog, click the Provenance icon.
• In the Provenance frame, click the info icon for one of the events
• Select the Content tab and click View. A new tab should open with the content viewer and the flowfile data can be viewed.
• Shut down NiFi and the proxy.

[mcgilman]

Reviewing...

[mcgilman]

Thanks @jtstorck! This has been merged to master.