http*_proxy improvements

- HOST_MACHINE can be used in http*_proxy fields
  - expands to host machine's 'hostname -f' output
  - helps enable developers running local caches
- proxy health checking: if the proxy isn't up,
  it won't be used. grumpy dev doesn't want to
  install a proxy… fine. not up? no problem.
- install.sh is sed'd to rewrite https link to http
  so we can cache the requests.
  - seems to get what's needed (despite only changing
    the first link that's redirected immediately).
- decreases test-kitchen run from 20m to 2m on
  cable modem.