Add SessionProvisioner and InstanceProvisionerFactory

Remove AllowedImages and IsDockerHost as it is not really being used for
anything useful

provisioner/factory.go

@@ -0,0 +1,18 @@
+package provisioner
+
+type instanceProvisionerFactory struct {
+	windows InstanceProvisionerApi
+	dind    InstanceProvisionerApi
+}
+
+func NewInstanceProvisionerFactory(w InstanceProvisionerApi, d InstanceProvisionerApi) InstanceProvisionerFactoryApi {
+	return &instanceProvisionerFactory{windows: w, dind: d}
+}
+
+func (p *instanceProvisionerFactory) GetProvisioner(instanceType string) (InstanceProvisionerApi, error) {
+	if instanceType == "windows" {
+		return p.windows, nil
+	} else {
+		return p.dind, nil
+	}
+}