ipadnsconfig: Allow execution of plugin in client host.

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

A new test playbook can be found at:

    tests/dnsconfig/test_dnsconfig_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.

tests/dnsconfig/test_dnsconfig_client_context.yml

@@ -0,0 +1,37 @@
+---
+- name: Test dnsconfig
+  hosts: ipaclients, ipaserver
+  become: no
+  gather_facts: no
+
+  tasks:
+  - name: Include FreeIPA facts.
+    include_tasks: ../env_freeipa_facts.yml
+
+  # Test will only be executed if host is not a server.
+  - name: Execute with server context in the client.
+    ipadnsconfig:
+      ipaadmin_password: SomeADMINpassword
+      ipaapi_context: server
+      forward_policy: none
+    register: result
+    failed_when: not (result.failed and result.msg is regex("No module named '*ipaserver'*"))
+    when: ipa_host_is_client
+
+# Import basic module tests, and execute with ipa_context set to 'client'.
+# If ipaclients is set, it will be executed using the client, if not,
+# ipaserver will be used.
+#
+# With this setup, tests can be executed against an IPA client, against
+# an IPA server using "client" context, and ensure that tests are executed
+# in upstream CI.
+
+- name: Test dnsconfig using client context, in client host.
+  import_playbook: test_dnsconfig.yml
+  when: groups['ipaclients']
+  vars:
+    ipa_test_host: ipaclients
+
+- name: Test dnsconfig using client context, in server host.
+  import_playbook: test_dnsconfig.yml
+  when: groups['ipaclients'] is not defined or not groups['ipaclients']