local-config element adds authorized_key for root:

Currently, the local-config and stackuser elements
are intertwined.  local-config installs an authorized_key
for stackuser, if the stackuser element is in use.

This change does two things:
- add authorized_key for root from local-config element,
    regardeless of whether stackuser element is in use.
- install key for stack user from stackuser element,
    only if local-config element is in use.

Change-Id: I0d07b61404119ea0650c5c0fb98d6786adcf3ca9