update logic for tag-setting to be Chef-specific only

- simplified `get_combined_tags`
  - no longer needs hostname, as it only needs Chef::Node object now
  - pass along the 'chef' source type to enable `tag:sometag` in Datadog
- renamed `host_tags` to `chef_env`, since it no longer looks up current host tags
- dropped `get_host_tags` methods
- updated test suite to ensure that no `GET` calls for tags are made
- re-recorded VCR cassettes for added `source` parameter

Fixes #28 
Refs DataDog/chef-datadog#78

The bulk of the change is in `lib/chef/handler/datadog.rb`.