RBAC Migration Followup #17: Discovery based gating

[enj]

Use discovery API based gating instead of hard coding version numbers.

[enj]

We currently hard code the version number for the gated commands at 3.7.0 (see pkg/cmd/util/clientcmd/gating.go and its callers). A better approach would be to do a discovery call against the API server, and determine if it supports the legacy policy resources. If it does, then we know its safe to run the commands. If it does not, then we know the cluster is too new. This could be useful for other commands in the future.

@openshift/cli-review any thoughts on this?

[enj]

@simo5 based on a conversation with @jupierce, we actually need to do discovery based gating because we have internal clusters that will be upgraded from 3.7 (pre-RBAC) to 3.7 (post-RBAC). I suppose we could do hacks to make version based gating work, but using discovery seems like the correct approach.