Support for a Flow to start from direct URL path access

Support for a flow to start when user directly access the flow path from an URL.

If a user points to [http://server/app/flow-folder/](http://server/app/flow-folder/) the implementation throws an exception saying it couldn't find an index.xhtml page.

Right now, the only way to start a flow is by having the user accessing another page before, outside of the flow, and then an action to the flow (either by link, button, or some javascript to automatically click on something).
#### Affected Versions
[2.2]

Start by reproducing direct access to http://server/app/flow-folder/ and trace the flow entry-point handling that reports the missing index.xhtml page. Done means a user can open the flow-folder URL directly and the flow starts without first visiting another page.

Written by the indexing model from the issue text.