Add ability to encrypt CA private key at rest (#386)

Fixes #8.

`nebula-cert ca` now supports encrypting the CA's private key with a
passphrase. Pass `-encrypt` in order to be prompted for a passphrase.
Encryption is performed using AES-256-GCM and Argon2id for KDF. KDF
parameters default to RFC recommendations, but can be overridden via CLI
flags `-argon-memory`, `-argon-parallelism`, and `-argon-iterations`.

cmd/nebula-cert/passwords_test.go

@@ -0,0 +1,10 @@
+package main
+
+type StubPasswordReader struct {
+	password []byte
+	err      error
+}
+
+func (pr *StubPasswordReader) ReadPassword() ([]byte, error) {
+	return pr.password, pr.err
+}