allow setting spec.upgradable on in-use kdapps

[joel-bluedata]

Part of issue #588. Depends on issue #589, which should be closed before these changes here are added.

This is mostly so that existing kdclusters for "old" kdapps can be marked to take advantage of live app upgrade IF it is deemed that the old kdapp doesn't need to respond to any upgrade notifications.

We can change KubeDirectorAppSpec.Upgradable to a pointer type, so we can tell when it is unspecified. (And code that looks at this field should treat unspecified as false.) The kdapp change validator can allow setting this field, even for in-use kdapps, if it is currently unspecified.