Add support for top level role groupings

[astubbs]

I have to work with ansible play books that already have roles defined, e.g. "kafka" - it expects inventory with a group of hosts under the "kafka" label.

The closest i can get is "role_kafka" by adding a tag role = kafka.

Perhaps a special reserved key name for the tag of "role" could be treated differently?

[astubbs]

Added a POC for this in #118 . Works for me so far.