install sudo in the devuser element

The devuser element can set up passwordless sudo, which requiers the
/etc/sudoers.d directory, which requires the sudo package, so we ensure
the sudo package is installed.

Change-Id: I80d6c669d4ac0d97b49d01cb621bf05b8e7f8ef1

diskimage_builder/elements/devuser/element-deps

@@ -0,0 +1 @@
+package-installs

diskimage_builder/elements/devuser/package-installs.yaml

@@ -0,0 +1 @@
+sudo:

diskimage_builder/elements/devuser/pkg-map

@@ -0,0 +1,7 @@
+{
+  "family": {
+    "gentoo": {
+      "sudo": "app-admin/sudo"
+    }
+  }
+}