tests: Fix AttributeError in callback plugins used by test suite

[moreati]

CALLBACK_VERSION et al are documented as required in
https://docs.ansible.com/ansible/2.10/dev_guide/developing_plugins.html#callback-plugins.
The need for document_fragment is noted in
https://github.com/ansible/ansible/blob/cfa8075537f5fa83ca5bfe3170373aaef9ce932f/lib/ansible/plugins/callback/default.py#L28-L32

Fixes #758

This addresses the following error, seen while running
ansible_tests.py.

TASK [Gathering Facts gather_timeout=10, gather_subset=['all']]
****************
task path:
/home/alex/src/mitogen/tests/ansible/regression/issue_109__target_has_old_ansible_installed.yml:4
[WARNING]: Failure using method (v2_runner_on_start) in callback plugin
(<ansible.plugins.callback.nice_stdout.CallbackModule object at
0x7f76b3dad090>): 'show_per_host_start'
Callback Exception:
File
"/home/alex/src/mitogen/.tox/py27-ansible2.10/lib/python2.7/site-packages/ansible/executor/task_queue_manager.py",
line 372, in send_callback
    method(*new_args, **kwargs)
File
"/home/alex/src/mitogen/.tox/py27-ansible2.10/lib/python2.7/site-packages/ansible/plugins/callback/default.py",
line 240, in v2_runner_on_start
    if self.get_option('show_per_host_start'):
File
"/home/alex/src/mitogen/.tox/py27-ansible2.10/lib/python2.7/site-packages/ansible/plugins/callback/__init__.py",
line 91, in get_option
    return self._plugin_options[k]
Callback Exception:
File
"/home/alex/src/mitogen/.tox/py27-ansible2.10/lib/python2.7/site-packages/ansible/executor/task_queue_manager.py",
line 372, in send_callback
    method(*new_args, **kwargs)
File
"/home/alex/src/mitogen/.tox/py27-ansible2.10/lib/python2.7/site-packages/ansible/plugins/callback/default.py",
line 240, in v2_runner_on_start
    if self.get_option('show_per_host_start'):
File
"/home/alex/src/mitogen/.tox/py27-ansible2.10/lib/python2.7/site-packages/ansible/plugins/callback/__init__.py",
line 91, in get_option
    return self._plugin_options[k]
[task 339882] 00:00:08.172036 D ansible_mitogen.affinity: CPU mask for
WorkerProcess: 0x000004
Callback Exception:
File
"/home/alex/src/mitogen/.tox/py27-ansible2.10/lib/python2.7/site-packages/ansible/executor/task_queue_manager.py",
line 372, in send_callback
    method(*new_args, **kwargs)
File
"/home/alex/src/mitogen/.tox/py27-ansible2.10/lib/python2.7/site-packages/ansible/plugins/callback/default.py",
line 240, in v2_runner_on_start
    if self.get_option('show_per_host_start'):
File
"/home/alex/src/mitogen/.tox/py27-ansible2.10/lib/python2.7/site-packages/ansible/plugins/callback/__init__.py",
line 91, in get_option
    return self._plugin_options[k]

Thanks for creating a PR! Here's a quick checklist to pay attention to:

• Please add an entry to docs/changelog.rst as appropriate.

• Has some new parameter been added or semantics modified somehow? Please
  ensure relevant documentation is updated in docs/ansible.rst and
  docs/api.rst.

• If it's for new functionality, is there at least a basic test in either
  tests/ or tests/ansible/ covering it?

• If it's for a new connection method, please try to stub out the
  implementation as in tests/data/stubs/, so that construction can be tested
  without having a working configuration.