Refactor utils that deal with certs

- merge `pkg/util/{crypto,certificates}`
- add funcs from `github.com/kubernetes-incubator/bootkube/pkg/tlsutil`
- ensure naming of funcs is fairly consistent

hack/.linted_packages

@@ -156,9 +156,8 @@ pkg/storage
 pkg/storage/etcd3
 pkg/storage/storagebackend/factory
 pkg/util/async
-pkg/util/certificates
+pkg/util/cert
 pkg/util/codeinspector
-pkg/util/crypto
 pkg/util/flock
 pkg/util/flushwriter
 pkg/util/goroutinemap