engine: renamed ImageType to ImageFileType

Renamed, because the ImageType conflicts with the upcomning
Instance Type feature, where there will be this entities:
ImageType
InstanceType
TemplateType

Change-Id: I5e72e98776631755e5c0296fa5c255118fb39a54
Signed-off-by: Tomas Jelinek <tjelinek@redhat.com>