ipaautomountmap: add support for indirect maps

Indirect maps were not supported by ansible-freeipa ipaautomountmap.
This patch adds support for adding indirect automount maps using the
"parent" and "mount" parameters, if the map do not yet exist. An
existing map cannot be modified.

The "parent" parameter must match an existing automount map, and the
"mount" parameter is required if "parent" is used.

A new example playbook can be found at:

    playbooks/automount/automount-map-indirect-map.yml

A new test playbook was added to test the feature:

    tests/automount/test_automountmap_indirect.yml