Make user value for GlanceImageImportPlugin prevail on logic
Make user value for GlanceImageImportPlugin prevail on logic

We used to *append* image_conversion to the list of enabled Glance
import plugins when both Glance and Nova were configured to use RBD
but we should instead make user input prevail on the logic, assuming
users who customize the THT param know what they are doing and
eventually include image_conversion only when they need it.

Change-Id: I5c0788bb361323f5a0e69e22d4bd42fa3c1e06ed

openstack/tripleo-heat-templates,openstack/tripleo-heat-templates