(#8644) Host provider on Windows

The host provider did not work on Windows because it didn't know where
to find its hosts file. The provider now uses Win32::Resolv, which is
part of the standard ruby library, to find it.

Several host type/provider spec tests were marked as fails_on_windows,
but now that the provider is working, I removed the tag from those
tests, and verified that the tests now pass. There are two tests in
resources_spec that fail because the user and exec providers are not
supported on Windows yet, so those tests are marked as fails_on_windows.

Reviewed-by: Pieter van de Bruggen <pieter@puppetlabs.com>