New IPADNSZone module

There is a new management module placed in the plugins folder:

    plugins/modules/ipadnszone.py

    The dnszone module allows to manage DNS zones.

    Here is the documentation for the module:

    README-dnszone.md

    New example playbooks have been added:

    playbooks/dnszone/disable-zone-forwarders.yml
    playbooks/dnszone/dnszone-absent.yml
    playbooks/dnszone/dnszone-all-params.yml
    playbooks/dnszone/dnszone-disable.yml
    playbooks/dnszone/dnszone-enable.yml
    playbooks/dnszone/dnszone-present.yml

    New tests for the module:

    tests/dnszone/test_dnszone.yml
    tests/dnszone/test_dnszone_mod.yml

plugins/modules/README.md

@@ -0,0 +1,80 @@
+# Writing a new Ansible FreeIPA module
+
+## Minimum requirements
+A ansible-freeipa module should have:
+
+* Code:
+  * A module file placed in `plugins/modules/<ipa_module_name>.py`
+
+* Documentation:
+  * `README-<module_name>.md` file in the root directory and linked from the main README.md
+  * Example playbooks in `playbooks/<module_name>/` directory
+
+* Tests:
+  * Test cases (also playbooks) defined in `tests/<module_name>/test_<something>.yml`. It's ok to have multiple files in this directory.
+
+## Code
+
+The module file have to start with the python shebang line, license header and definition of the constants `ANSIBLE_METADATA`, `DOCUMENTATION`, `EXAMPLES` and `RETURNS`. Those constants need to be defined before the code (even imports). See https://docs.ansible.com/ansible/latest/dev_guide/developing_modules_general.html#starting-a-new-module for more information.
+
+
+Although it's use is not yet required, ansible-freeipa provides `FreeIPABaseModule` as a helper class for the implementation of new modules. See the example bellow:
+
+```python
+
+from ansible.module_utils.ansible_freeipa_module import FreeIPABaseModule
+
+
+class SomeIPAModule(FreeIPABaseModule):
+    ipa_param_mapping = {
+        "arg_to_be_passed_to_ipa_command": "module_param",
+        "another_arg": "get_another_module_param",
+    }
+
+    def get_another_module_param(self):
+        another_module_param = self.ipa_params.another_module_param
+
+        # Validate or modify another_module_param ...
+
+        return another_module_param
+
+    def check_ipa_params(self):
+
+        # Validate your params here ...
+
+        # Example:
+        if not self.ipa_params.module_param in VALID_OPTIONS:
+            self.fail_json(msg="Invalid value for argument module_param")
+
+    def define_ipa_commands(self):
+        args = self.get_ipa_command_args()
+
+        self.add_ipa_command("some_ipa_command", name="obj-name", args=args)
+
+
+def main():
+    ipa_module = SomeIPAModule(argument_spec=dict(
+        module_param=dict(type="str", default=None, required=False),
+        another_module_param=dict(type="str", default=None, required=False),
+    ))
+    ipa_module.ipa_run()
+
+
+if __name__ == "__main__":
+    main()
+```
+
+In the example above, the module will call the command `some_ipa_command`, using "obj-name" as name and, `arg_to_be_passed_to_ipa_command` and `another_arg` as arguments.
+
+The values of the arguments will be determined by the class attribute `ipa_param_mapping`.
+
+In the case of `arg_to_be_passed_to_ipa_command` the key (`module_param`) is defined in the module `argument_specs` so the value of the argument is actually used.
+
+On the other hand, `another_arg` as mapped to something else: a callable method. In this case the method will be called and it's result used as value for `another_arg`.
+
+**NOTE**: Keep mind that to take advantage of the parameters mapping defined in `ipa_param_mapping` you will have to call `args = self.get_ipa_command_args()` and use `args` in your command. There is no implicit call of this method.
+
+
+## Disclaimer
+
+The `FreeIPABaseModule` is new and might not be suitable to all cases and every module yet. In case you need to extend it's functionality for a new module please open an issue or PR and we'll be happy to discuss it.