Change: Enterprise HA policy cleanup

Ref: https://dev.cfengine.com/issues/6813

The CFEngine Enterpise HA policy uses features from 3.6. When the policy is
loaded on 3.5 and earlier clients it fails to validate.

Make the HA policy only load on En Additionally there is no reason to load the
HA policy on clients that are not enterprise hubs.

Change class names to make them more self describing and specific to reduce
changes of clashing with custom policies.

Move enablement of hub ha to def and update_def from ha_def with other feature
enablement classes.