OMERO5.1DEV__14: add "ldap" to experimentergroup.
Update tests to set correct model object fields.

After a general review of usages of Experimenter and
ExperimenterGroup instances, all of them have been updated
to set the "ldap" field to false to avoid Hibernate exceptions.
Set "ldap" field on group creation.

This guarantees that the only way to add/remove the LDAP flag
for groups is the "omero ldap" CLI plugin. The Webclient will
automatically set "ldap" to false for newly created groups.
Updated ones will keep their previous "ldap" value.
{user,group}.py: Display "ldap" in "list".
Add findGroupDN() API method.

This allows for looking up DNs for groups. OmeroModelContextMapper
has been created as an abstract class encapsulating the common
state used by both the user and group context mappers. This logic
lacks integration tests.
Deprecate setDN().

This change is dictated by the fact, that Experimenter can
now control the "ldap" flag by setLdap(). The only place where
setDN() is still used is the Java LDAP integration test, but
that can be easily updated.
ldap.py: setdn, getdn, discover work for groups.

This commit adds two new API methods required for the
mentioned CLI commands to work. Some of the code could be
optimised further as it is similar to the user-handling
API logic.