Clean up use of global conf vars in start_nova

Make it clear which services run in the API cell and compute (child)
cells by using appropriately named local variables for the conf files.
This should help save from future bugs.

Also: When cells is enabled, there's no need to run nova-conductor in
the API cell right now.  Cells bypasses any use of conductor in the API
cell.

Change-Id: I3af17d3db028f5df36814cb83c7db4de8f141f84
Merge "Clean up use of global conf vars in start_nova"