Modify supplemental apt repo setup

Fix bug 1214145 by doing two things:
* Add the signing key for the supplemental repo to the apt::source
  definition.  This prevents attempts to fetch the key based on
  ID from a keyserver, even though it's the same key used in the
  main repo.
* Make the supplemental repo optional and allow users to specify
  it's location.  The supplemental repo is currently not needed for
  many installations and it is not necessary to configure it for
  a functional cloud.  Users with no-net install scenarios may
  also wish to point to a local mirror or cache rather than the
  real thing if they need it at all.  This commit allows users
  to choose to not set up the supplemental repo at all by commenting
  out one line in site.pp or to change the location from the
  defaults.

Fixes: bug #1214145
Merge pull request #154 from CiscoSystems/bugs/1214145

Modify supplemental apt repo setup