kustomize lookup plugin: build_flags

Allow setting a string of optional build_flags at the end of the
command.

plugins/lookup/kustomize.py

@@ -30,6 +30,9 @@
       opt_dirs:
         description:
         - An optional list of directories to search for the executable in addition to PATH.
+      build_flags:
+        description:
+        - Set additional build flags, like --enable-alpha-plugins
 
     requirements:
       - "python >= 3.6"
@@ -136,6 +139,9 @@ def run(
                 )
             )
 
+        if "build_flags" in kwargs:
+            command += [kwargs["build_flags"]]
+
         (out, err) = run_command(command)
         if err:
             raise AnsibleLookupError(