Use buildah commit for podman commit

Resolves: #586 and #520
Signed-off-by: baude <bbaude@redhat.com>

Closes: #592
Approved by: mheon

libpod/buildah/common.go

@@ -0,0 +1,28 @@
+package buildah
+
+import (
+	"io"
+
+	cp "github.com/containers/image/copy"
+	"github.com/containers/image/types"
+)
+
+func getCopyOptions(reportWriter io.Writer, sourceSystemContext *types.SystemContext, destinationSystemContext *types.SystemContext, manifestType string) *cp.Options {
+	return &cp.Options{
+		ReportWriter:          reportWriter,
+		SourceCtx:             sourceSystemContext,
+		DestinationCtx:        destinationSystemContext,
+		ForceManifestMIMEType: manifestType,
+	}
+}
+
+func getSystemContext(defaults *types.SystemContext, signaturePolicyPath string) *types.SystemContext {
+	sc := &types.SystemContext{}
+	if defaults != nil {
+		*sc = *defaults
+	}
+	if signaturePolicyPath != "" {
+		sc.SignaturePolicyPath = signaturePolicyPath
+	}
+	return sc
+}