If you are using environments/dcn-hci.yaml, then you very likely
have more than one Glance server and will want to use the copy-image
feature. Thus, enable it by default for deployments which use this
environment file.
Also because GlanceCacheEnabled defaults to False and because
GlanceImageCacheMaxSize defaults to 10737418240 we don't need
to explicitly set them in environments/dcn{,-hci}.yaml.
Change-Id: If745aa0824098950367525170eaf6cb4e3804482
161071f2d8