Vendor after merging mtrmac/image:openpgp, + other updates

Primarily vendor after merging mtrmac/image:openpgp.

Then update for the SigningMechanism API change.

Also skip signing tests if the GPG mechanism does not support signing.

Also abort some of the tests early instead of trying to use invalid (or
nil) values.

The current master of image-tools does not build with Go 1.6, so keep
using an older release.

Also requires adding a few more dependencies of our updated
dependencies.

vendor/github.com/opencontainers/selinux/go-selinux/label/label.go

@@ -0,0 +1,84 @@
+// +build !selinux !linux
+
+package label
+
+// InitLabels returns the process label and file labels to be used within
+// the container.  A list of options can be passed into this function to alter
+// the labels.
+func InitLabels(options []string) (string, string, error) {
+	return "", "", nil
+}
+
+func GetROMountLabel() string {
+	return ""
+}
+
+func GenLabels(options string) (string, string, error) {
+	return "", "", nil
+}
+
+func FormatMountLabel(src string, mountLabel string) string {
+	return src
+}
+
+func SetProcessLabel(processLabel string) error {
+	return nil
+}
+
+func GetFileLabel(path string) (string, error) {
+	return "", nil
+}
+
+func SetFileLabel(path string, fileLabel string) error {
+	return nil
+}
+
+func SetFileCreateLabel(fileLabel string) error {
+	return nil
+}
+
+func Relabel(path string, fileLabel string, shared bool) error {
+	return nil
+}
+
+func GetPidLabel(pid int) (string, error) {
+	return "", nil
+}
+
+func Init() {
+}
+
+func ReserveLabel(label string) error {
+	return nil
+}
+
+func ReleaseLabel(label string) error {
+	return nil
+}
+
+// DupSecOpt takes a process label and returns security options that
+// can be used to set duplicate labels on future container processes
+func DupSecOpt(src string) []string {
+	return nil
+}
+
+// DisableSecOpt returns a security opt that can disable labeling
+// support for future container processes
+func DisableSecOpt() []string {
+	return nil
+}
+
+// Validate checks that the label does not include unexpected options
+func Validate(label string) error {
+	return nil
+}
+
+// RelabelNeeded checks whether the user requested a relabel
+func RelabelNeeded(label string) bool {
+	return false
+}
+
+// IsShared checks that the label includes a "shared" mark
+func IsShared(label string) bool {
+	return false
+}