A ReferenceGrant Should Be Removed From Watchable Map When UnReferenced

[danehans]

#471 added initial support for ReferenceGrants by allowing Gateways to reference Secrets in a different namespace. The gateway controller adds the ReferenceGrant and the ReferenceGrant namespace to the provider watchable map when:

1. A Gateway listener references a Secret in a different namespace.
2. A ReferenceGrant in the above namespace exists and permits the Gateways from the Gateway's namespace to Secrets.

When the Gateway is deleted, the ReferenceGrant and ReferenceGrant namespace should be deleted from the provider's watchable map if no other Gateway references the ReferenceGrant.

[skriss]

Note #580 (review), when this is fixed the two invalid/missing referencegrant conformance tests should be able to be uncommented.

[arkodg]

I prefer if we move this to 0.3.0 . Based on current logic there is no easy way to handle deletes for RefGrants, easier to implement this with #413 where we dont need to handle deletes, just start from scratch with every reconcile.

[danehans]

Being noted in the v0.2.0 release notes and moving to v0.3.0-rc.1.