iparole: Allow execution of plugin in client host.

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

A new test playbook can be found at:

    tests/role/test_role_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/role/env_cleanup.yml

@@ -2,6 +2,7 @@
 - name: Ensure test user is absent.
   ipauser:
     ipaadmin_password: SomeADMINpassword
+    ipaapi_context: "{{ ipa_context | default(omit) }}"
     name:
     - user01
     - user02
@@ -11,6 +12,7 @@
 - name: Ensure test group is absent.
   ipagroup:
     ipaadmin_password: SomeADMINpassword
+    ipaapi_context: "{{ ipa_context | default(omit) }}"
     name:
     - group01
     - group02
@@ -19,6 +21,7 @@
 - name: Ensure test hostgroup is absent.
   ipahostgroup:
     ipaadmin_password: SomeADMINpassword
+    ipaapi_context: "{{ ipa_context | default(omit) }}"
     name:
     - hostgroup01
     - hostgroup02
@@ -27,6 +30,7 @@
 - name: Ensure test host is absent.
   ipahost:
     ipaadmin_password: SomeADMINpassword
+    ipaapi_context: "{{ ipa_context | default(omit) }}"
     name:
     - "{{ host1_fqdn }}"
     - "{{ host2_fqdn }}"
@@ -35,6 +39,7 @@
 - name: Ensure test service is absent.
   ipaservice:
     ipaadmin_password: SomeADMINpassword
+    ipaapi_context: "{{ ipa_context | default(omit) }}"
     name:
     - "service01/{{ host1_fqdn }}"
     - "service02/{{ host2_fqdn }}"
@@ -43,6 +48,7 @@
 - name: Ensure test roles are absent.
   iparole:
     ipaadmin_password: SomeADMINpassword
+    ipaapi_context: "{{ ipa_context | default(omit) }}"
     name:
     - renamerole
     - testrole