Modified docs to update DevStack's config filename

As reported in the bug, the documentation needs to be updated with
new config file DevStack uses. This patch simply updates information
in "Installing and Running the Development Version" page.

Change-Id: I5ee8f528be44b4fe801b38c748fff7314980a93e
Closes-Bug: #1370476
Merge "Modified docs to update DevStack's config filename"