Add usage type back

Signed-off-by: Sean Holcomb <seanholcomb@gmail.com>

core/pkg/model/shared/usagetype.go

@@ -0,0 +1,10 @@
+package shared
+
+// TODO remove this?
+type UsageType string
+
+const (
+	UsageTypeEmpty    UsageType = ""
+	UsageTypeOnDemand UsageType = "OnDemand"
+	UsageTypeSpot     UsageType = "Spot"
+)