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.
