enhancements to install_packages macro

- Adds debian_package_install method to the jinja environment
  that will return the necessary set of commands needed to
  install packages via apt-get and/or URLs to .deb packages
- Updates install_packages macro to make use of the method
  above

Change-Id: Ie9318a7def54b5034ba91375fd8fd3b589d18349
Partially-implements: blueprint third-party-plugin-support