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.
vndr is almost exactly the same as our old good hack/vendor.sh. Except
it's cleaner and it allows to re-vendor just one dependency if needed
(which we do a lot for containers/image).
Signed-off-by: Antonio Murdaca <runcom@redhat.com>
Otherwise the "clean" step of hack/vendor.sh would drop most .go files
from vendor/ as unused.
Also commits refreshed versions of a few of the vendored packages.
