ipasudorule: Allow execution of plugin in client host.

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

A new test playbook can be found at:

    tests/sudorule/test_sudorule_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.

README-sudorule.md

@@ -120,6 +120,7 @@ Variable | Description | Required
 -------- | ----------- | --------
 `ipaadmin_principal` | The admin principal is a string and defaults to `admin` | no
 `ipaadmin_password` | The admin password is a string and is required if there is no admin ticket available on the node | no
+`ipaapi_context` | The context in which the module will execute. Executing in a server context is preferred. If not provided context will be determined by the execution environment. Valid values are `server` and `client`. | no
 `name` \| `cn` | The list of sudorule name strings. | yes
 `description` | The sudorule description string. | no
 `usercategory` \| `usercat` | User category the rule applies to. Choices: ["all", ""] | no