Fix legacy nova/cinder encryption key manager configuration

Fix legacy nova/cinder encryption key manager configuration

Recent changes in Nova [0] and Cinder [1] result in Barbican being selected
as the default encryption key manager, even when TripleO is not deploying
Barbican.

This change ensures the legacy key manager is enabled when no key manager
(such as Barbican) has been specified. This restores the previous behavior,
where the legacy key manager was enabled by default.

[0] https://review.openstack.org/484501
[1] https://review.openstack.org/485322

Closes-Bug: #1706389
Change-Id: Idc92f7a77cde757538eaac51c4ad8dc397f9c3d3