[ldapfeed] Add support for LDAP groups (closes #2528116)
Groups from the LDAP server are imported as CWGourp entities, and in_group relationships are created between existing CWUsers and CWGroups
Unit tests have been refactored a bit, especially ti add helper methods to manage LDAP database.
debian/tmp/usr/lib/PY_VERSION/site-packages/cubicweb/etwist/ usr/lib/PY_VERSION/site-packages/cubicweb/