Add explicit Ubuntu support. Add Gentoo support.

[dbeckham]

Another example that you see used in most Puppetlabs modules:

case $::osfamily {
  'RedHat': {
    $foo = 'foo'
  }
  'Debian': {
    $foo = 'bar'
  }
  'SuSE': {
    $foo = 'baz'
  }
  default: {
    fail("Unsupported operating system $::osfamily/$::operatingsystem")
  }
}

At the moment, we are starting to make pull requests against puppet modules that we are using so that hopefully our bug fixes or Gentoo tweaks are useful for other ops teams. If you would prefer that Ubuntu be removed in this PR, I'm glad to commit another update if it would help move it forward. We already maintain a separate "production" branch that we make non backwards compatible changes in that are specific to our needs, so it's no problem for us.

[torrancew]

No worries, will merge it in as-is tonight when I have a breather.