Add CORE_TLS_SECRET configuration

This secret will be used to encrypt automatically obtained secrets at
rest, i.e. in a storage. They will be decrypted on demand. If the
secret is wrong, stored certificates can't be decrypted. For changing
the secret, the stored certificated must be deleted first in order
to obtain new ones that will be encrypted with the new secret.

slices/diff_test.go

@@ -0,0 +1,17 @@
+package slices
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestDiff(t *testing.T) {
+	a := []string{"c", "d", "e", "f"}
+	b := []string{"a", "b", "c", "d"}
+
+	added, removed := Diff(a, b)
+
+	require.Equal(t, []string{"e", "f"}, added)
+	require.Equal(t, []string{"a", "b"}, removed)
+}