lxd provider: import ubuntu-$series if it doesn't exist

Instead of making people import their own image, let's just do it for them
if it doesn't exist, like we do in the container type.

Signed-off-by: Tycho Andersen <tycho.andersen@canonical.com>

provider/lxd/environ_broker.go

@@ -95,6 +95,11 @@ func (env *environ) newRawInstance(args environs.StartInstanceParams) (*lxdclien
 	series := args.Tools.OneSeries()
 	image := "ubuntu-" + series
 
+	err := env.raw.EnsureImageExists(series)
+	if err != nil {
+		return nil, errors.Trace(err)
+	}
+
 	metadata, err := getMetadata(args)
 	if err != nil {
 		return nil, errors.Trace(err)

provider/lxd/environ_raw.go

@@ -16,6 +16,7 @@ import (
 type rawProvider struct {
 	lxdInstances
 	lxdProfiles
+	lxdImages
 	common.Firewaller
 	policyProvider
 }
@@ -32,6 +33,10 @@ type lxdProfiles interface {
 	HasProfile(string) (bool, error)
 }
 
+type lxdImages interface {
+	EnsureImageExists(series string) error
+}
+
 func newRawProvider(ecfg *environConfig) (*rawProvider, error) {
 	client, err := newClient(ecfg)
 	if err != nil {
@@ -48,6 +53,7 @@ func newRawProvider(ecfg *environConfig) (*rawProvider, error) {
 	raw := &rawProvider{
 		lxdInstances:   client,
 		lxdProfiles:    client,
+		lxdImages:      client,
 		Firewaller:     firewaller,
 		policyProvider: policy,
 	}