ipadnsforwardzone: Allow execution of plugin in client host.

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

A new test playbook can be found at:

    tests/dnsforwardzone/test_dnsforwardzone_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 ipadnsforwardzone in
a client context, some values had to be modified so that comparision
works, avoiding unnecessary IPA API calls.