Rename Apt environment variables to `VAGRANT_APT_HTTP_PROXY` form

Be consistent with all future environment variables:
  https://gist.github.com/tmatilai/6253077

The "VAGRANT_" prefix also makes it clear in which context they are
used.
Merge pull request #15 from tmatilai/apt-env-vars

Rename Apt environment variables to `VAGRANT_APT_HTTP_PROXY` form