Replace organization default values in LDAP. Refs #2931

The original nethserver-directory-user-modify has been diveded into
two actions:

* nethserver-directory-organization-modify: changes LDAP organization
  fields directly, and works with and without the "username"
  argument. Without username, the action updates all user accounts.

* nethserver-directory-user-modify: invokes lusermod on the given user.