automember: Add automember default group handling

The fallback group and hostgroup for unmached entries can be set and
unset using default_group. If default_group is empty, then the default
group will be unset.

DN and ipa_get_based provided by ansible_freeipa_module are used in the
code.

New parameters:
- default_group: Default (fallback) group for all unmatched entries.

New parameters and examples have been added to README-automember.md

New playbooks:
- playbooks/automember/automember-default-group-not-set.yml
- playbooks/automember/automember-default-group-set.yml
- playbooks/automember/automember-default-hostgroup-not-set.yml
- playbooks/automember/automember-default-hostgroup-set.yml

New tests:
- tests/automember/test_automember_default_group.yml