Clarify ResourceGroup properties support status

Add schema support_status tags to enable docs to reflect that:
- index_var is only available from Juno
- removal_policies will only be available from Kilo.

Hopefully, this can be a first step to documenting recently added
properties to reduce the FAQ-ness in IRC about them.

Change-Id: I334dfdc8c07f6e9411ae73152e601d8143808aeb
Merge "Clarify ResourceGroup properties support status"