ipavault: Allow execution of plugin in client host.

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

A new test playbook can be found at:

    tests/vault/test_vault_client_context.yml

As `ipavault` only works in client context, an error is raised if it
is explicitly executed in a server context.

tests/vault/env_setup.yml

@@ -35,11 +35,13 @@
   - name: Ensure vaultgroup exists.
     ipagroup:
       ipaadmin_password: SomeADMINpassword
+      ipaapi_context: "{{ ipa_context | default(omit) }}"
       name: vaultgroup
 
   - name: Ensure testing users exist.
     ipauser:
       ipaadmin_password: SomeADMINpassword
+      ipaapi_context: "{{ ipa_context | default(omit) }}"
       users:
       - name: user01
         first: First