core: use the ou=People dn for openldap users

In openldap deployments the ou=People dn is more widely used than
cn=Users,cn=Accounts (more freeipa-like).
Until we wait for this parameter to be configurable let's switch
it to ou=People.

Change-Id: I84707ad92bdd2427f9acb7974149859ed79a751a
Signed-off-by: Federico Simoncelli <fsimonce@redhat.com>