Infrastructure: Fix machine placement target defaults (#2151)

Make sure that the optional fields for the
VPCMachinePlacementTarget definition are configured properly
for JSON serialization.

Related: #2150

api/v1beta2/types.go

@@ -416,9 +416,10 @@ type VPCMachinePlacementTarget struct {
 
 	// DedicatedHostGroup defines the Dedicated Host Group to use when placing a VPC Machine (Instance).
 	// +optional
-	DedicatedHostGroup *VPCResource `json:"dedicatedHostGroup"`
+	DedicatedHostGroup *VPCResource `json:"dedicatedHostGroup,omitempty"`
 
 	// PlacementGroup defines the Placement Group to use when placing a VPC Machine (Instance).
+	// +optional
 	PlacementGroup *VPCResource `json:"placementGroup,omitempty"`
 }