The DNA magic value can be arbitrarily changed by admins so we cannot use a
const value to check. And we relly do not need to check at all. If the DNA
plugin is broken and leaves magic values to reach the post-op stage we have
bigger problems. So just simply get rid of this check.
A postop plugin is added to create the SID for new created users and
groups. A directory server task allows to set the SID for existing
users and groups.
Fixes https://fedorahosted.org/freeipa/ticket/2825
