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`.

cert/crypto_test.go

@@ -0,0 +1,25 @@
+package cert
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"golang.org/x/crypto/argon2"
+)
+
+func TestNewArgon2Parameters(t *testing.T) {
+	p := NewArgon2Parameters(64*1024, 4, 3)
+	assert.EqualValues(t, &Argon2Parameters{
+		version:     argon2.Version,
+		Memory:      64 * 1024,
+		Parallelism: 4,
+		Iterations:  3,
+	}, p)
+	p = NewArgon2Parameters(2*1024*1024, 2, 1)
+	assert.EqualValues(t, &Argon2Parameters{
+		version:     argon2.Version,
+		Memory:      2 * 1024 * 1024,
+		Parallelism: 2,
+		Iterations:  1,
+	}, p)
+}