Add a ubuntu package_repo option

Extend the package_repo logic to provide users three alternatives from
which they can obtain their OpenStack packages -- Ubuntu native packages
included in the base Ubuntu operating system, Canonical-supplied
packages distributed through the Ubuntu Cloud Archive PPA, or
Cisco-supplied packages distributed through a Cisco repository.

Closes-Bug: #1299193
Merge pull request #46 from CiscoSystems/bugs/1299193

Add a ubuntu package_repo option