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/openshift/imagebuilder/constants.go

@@ -0,0 +1,13 @@
+package imagebuilder
+
+const (
+	// in docker/system
+	NoBaseImageSpecifier = "scratch"
+
+	// not yet part of our import
+	commandArg        = "arg"
+	commandStopSignal = "stopsignal"
+
+	// in docker/system
+	defaultPathEnv = "/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin"
+)