ipadnsrecord: Allow execution of plugin in client host.

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

A new test playbook can be found at:

    tests/dnsrecord/test_dnsrecord_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/dnsrecord/env_setup.yml

@@ -10,6 +10,7 @@
   - name: Ensure DNS testing zones are present.
     ipadnszone:
       ipaadmin_password: SomeADMINpassword
+      ipaapi_context: "{{ ipa_context | default(omit) }}"
       name: "{{ item }}"
       skip_nameserver_check: yes
       skip_overlap_check: yes
@@ -25,6 +26,7 @@
   - name: Ensure DNSSEC zone '"{{ safezone }}"' is present.
     ipadnszone:
       ipaadmin_password: SomeADMINpassword
+      ipaapi_context: "{{ ipa_context | default(omit) }}"
       name: "{{ safezone }}"
       dnssec: yes
       skip_nameserver_check: yes