ipaconfig: Allow execution of plugin in client host.

Update config README file and add tests for executing plugin with
`ipaapi_context` set to `client`.

A new test playbook can be found at:

    tests/config/test_config_client_context.yml

The new test file can be executed in a FreeIPA client host that is
not a server. In this case, it should be defined in the `ipaclients`
group, in the inventory file.

As the tests for ipaconfig may render the server in an inconsistent
state if they fail, the tests in tests/config/test_config.yml were
wrapped in a way that if one test fails, the default FreeIPA
configuration is restored to the server.