vendor: switch to glide, update all dependencies

The validation logic in Ignition had diverged between here and the
latest release (v0.16.0) at some point. Since the vendored projects here
were already being touched, this commit took the opportunity to switch
everything here to use glide and update all dependencies to the latest
version.

The new Makefile has a vendor target that runs the correct glide
commands to reapply any changes to glide.yaml.

Author: Derek Gonyeo

Makefile

@@ -0,0 +1,4 @@
+.PHONY: vendor
+vendor:
+	@glide update --strip-vendor
+	@glide-vc --use-lock-file --no-tests --only-code

glide.lock

@@ -0,0 +1,14 @@
+package: github.com/coreos/container-linux-userdata-validator
+import:
+- package: github.com/coreos/coreos-cloudinit
+  version: 1.13.0
+  subpackages:
+  - config/validate
+- package: github.com/coreos/ignition
+  version: 0.16.0
+  subpackages:
+  - config
+- package: github.com/crawford/nap
+  version: 0.1.0
+- package: github.com/gorilla/mux
+  version: 1.4.0