docs

All imports that are only available after installing IPA need to be in a
try exception clause to be able to pass the fake execution test. The old
workaround "if 'ansible.executor' in sys.modules:" is not working with
this test anymore.

If the imports can not be done, all used and needed attributes are
defines with the value None.

The new function check_imports has been added to fail with module.fail_json
if an import exception occured and ANSIBLE_IPA_CLIENT_MODULE_IMPORT_ERROR is
not None. This function needs to be called in all modules.

The `copyright` date is extended with `-2022`.