Permit use of sudo-ldap instead of sudo package

If the sudo-ldap package is providing the sudo command instead of the plain
sudo package, accept that instead of breaking the system and requiring direct
root login intervention to fix things.

Change-Id: I45d7e4617bd59e72b4f0bf2e91750a6830e2a010
diff --git a/stack.sh b/stack.sh
index fa5b43c..9b3097b 100755
--- a/stack.sh
+++ b/stack.sh
@@ -247,7 +247,7 @@
 # --------------
 
 # We're not as **root** so make sure ``sudo`` is available
-is_package_installed sudo || install_package sudo
+is_package_installed sudo || is_package_installed sudo-ldap || install_package sudo
 
 # UEC images ``/etc/sudoers`` does not have a ``#includedir``, add one
 sudo grep -q "^#includedir.*/etc/sudoers.d" /etc/sudoers ||