Default autoscaling policy rules are ineffective

There's a faulty logic in Assign.usePolicyFramework() that makes the default policy (added in SOLR-12845) ineffective - that is, in the absence of any user-provided modifications to the policy rules the code reverts to LEGACY assignment.

The logic in this method is convoluted and opaque, it's difficult for users to be sure what strategy is used when - instead we should make this choice explicit.

(BTW, the default ruleset is probably too expensive for large clusters anyway, given the unresolved performance problems in the policy engine).