Update the behaviour of "ldap discover".

The following changes update "ldap discover" to again
provide a list of LDAP users which are disabled in the
OMERO DB:
- Revert "Deprecate bin/omero ldap discover."
  This reverts commit 742f54d52656fa4cb35c9f57b825e664c7b1cacc.
- Change parameter name in setDN().
- Change return type of discover() and add test.
- Minor fixes in ldap.py.