Parameterize Cisco package repo setup

Previous versions of this module relied on static strings in the
late_command portion of the preseed template in order to set up
the Cisco package repositories in the preseed file passed to nodes
being PXE booted.  This meant that with every release, the
release manager would need to push a change to the file to set up
the repository properly for that release.  It also meant that
repository information specified for the puppet-coe module could
wind up being inconsistent with the information in the preseed,
thus generally violated DRY principles.  This patch parameterizes
the apt repo setup portion of the preseed.

Partial-Bug: #1269856

Note to reviewers: because fully fixing this bug requires touching multiple modules, a couple of other patches need to be reviewed and merged alongside of this one. I'll post those here as soon as I have them all submitted.

EDIT: other reviews:
https://github.com/CiscoSystems/puppet-coe/pull/35
https://github.com/CiscoSystems/puppet-coi/pull/38
https://review.openstack.org/67916