How to create groups for device_role * site in the inventory plugin?

[ccope]

I'm trying to fetch all of the machines matching a set of roles and sites, and then make site-specific groups. Ex:

query_filters:
  - role: role1
  - role: role2
  - role: role3
  - site: site1
  - site: site2
group_by:
 - ???

such that I get the following groups:

• site1_role1
• site2_role1
• site1_role2
• site2_role2
• site1_role3
• site2_role3

Is this possible?