ipadnszone: Allow execution of plugin in client host.

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

A new test playbook can be found at:

    tests/dnszone/test_dnszone_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.

Due to differences in data returned when running in a client context,
verification if zone is active had to be modified.

tests/dnszone/env_cleanup.yml

@@ -2,6 +2,7 @@
 - name: Ensure zone is absent.
   ipadnszone:
     ipaadmin_password: SomeADMINpassword
+    ipaapi_context: "{{ ipa_context | default(omit) }}"
     name:
       - testzone.local
       - test1.testzone.local