vendor: Bump to include containers/image

And zounds of other dependencies.  Use Gopkg.toml's 'ignored' [1] to
exclude, BoltDB, which comes in via:

  github.com/containers/image/pkg/blobinfocache
    github.com/boltdb/bolt

and github.com/docker/distribution/registry/storage/cache, which comes
in via:

  github.com/containers/image/docker
    github.com/docker/distribution/registry/client
      github.com/docker/distribution/registry/storage/cache

because we don't use either the storage backend or the BoltDB
blob-info cache, but dep isn't checking at that level of granularity.
Ideally the upstream repositories would restructure to split these out
into separate packages (and/or dep will grow support for pruning by
build tag [2]), but until then, we can manually prune via 'ignored'.

Also ignore mtrmac/gpgme, because we only need signature verification
and we don't want to use CGO (because we want to be portable to other
operating systems, and we only need verification support, not signing
support [3]).

There may be more fat we can prune as well, but I got tired of looking
and gave up, even though ~50k lines of new code is pretty embarassing
for what is effectively just a handful of HTTPS requests and an
OpenPGP check.

Generated (after manually editing Gopkg.toml) with:

  $ dep ensure

using:

  $ dep version
  dep:
   version     : v0.5.0-31-g73b3afe
   build date  : 2019-02-08
   git hash    : 73b3afe
   go version  : go1.10.3
   go compiler : gc
   platform    : linux/amd64
   features    : ImportDuringSolve=false

[1]: https://golang.github.io/dep/docs/Gopkg.toml.html#ignored
[2]: golang/dep#291
[3]: containers/image#268

Gopkg.toml

@@ -1,6 +1,9 @@
 ignored = [
-  "github.com/openshift/installer/tests*",
-  "github.com/openshift/installer/pkg/terraform/exec*"
+  "github.com/boltdb/*",
+  "github.com/docker/distribution/metrics",
+  "github.com/mtrmac/gpgme",
+  "github.com/openshift/installer/pkg/terraform/exec*",
+  "github.com/openshift/installer/tests*"
 ]
 
 [prune]

hack/build.sh

@@ -32,7 +32,7 @@ fi
 
 MODE="${MODE:-release}"
 LDFLAGS="${LDFLAGS} -X github.com/openshift/installer/pkg/version.Raw=$(git describe --always --abbrev=40 --dirty)"
-TAGS="${TAGS:-}"
+TAGS="${TAGS:-} containers_image_openpgp"
 OUTPUT="${OUTPUT:-bin/openshift-install}"
 export CGO_ENABLED=0