Use buildah commit and bud in podman

Vendor in buildah and use as much of commit and bug as possible for podman
build and commit.

Resolves #586
Signed-off-by: baude <bbaude@redhat.com>

Closes: #681
Approved by: mheon

vendor/github.com/projectatomic/buildah/unmount.go

@@ -0,0 +1,11 @@
+package buildah
+
+// Unmount unmounts a build container.
+func (b *Builder) Unmount() error {
+	err := b.store.Unmount(b.ContainerID)
+	if err == nil {
+		b.MountPoint = ""
+		err = b.Save()
+	}
+	return err
+}