ipatrust: Allow execution of plugin in client host.

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

A new test playbook can be found at:

    tests/trust/test_trust_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-trust.md

@@ -101,6 +101,7 @@ 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
 `realm` | The realm name string. | yes
 `admin` | Active Directory domain administrator string. | no
 `password` | Active Directory domain administrator's password string. | no

tests/trust/test_trust.yml

 ---
 - name: find trust
-  hosts: ipaserver
+  hosts: "{{ ipa_test_host | default('ipaserver') }}"
   become: true
   gather_facts: false
 
@@ -11,6 +11,7 @@
     - name: delete trust
       ipatrust:
         ipaadmin_password: SomeADMINpassword
+        ipaapi_context: "{{ ipa_context | default(omit) }}"
         realm: windows.local
         state: absent
       register: del_trust
@@ -38,6 +39,7 @@
     - name: add trust
       ipatrust:
         ipaadmin_password: SomeADMINpassword
+        ipaapi_context: "{{ ipa_context | default(omit) }}"
         realm: windows.local
         admin: Administrator
         password: secret_ad_pw

tests/trust/test_trust_client_context.yml

+---
+- name: Test trust
+  hosts: ipaclients, ipaserver
+  become: no
+  gather_facts: no
+
+  tasks:
+  - name: Include FreeIPA facts.
+    include_tasks: ../env_freeipa_facts.yml
+
+  # Test will only be executed if host is not a server.
+  - name: Execute with server context in the client.
+    ipatrust:
+      ipaadmin_password: SomeADMINpassword
+      ipaapi_context: server
+      realm: windows.local
+    register: result
+    failed_when: not (result.failed and result.msg is regex("No module named '*ipaserver'*"))
+    when: ipa_host_is_client
+
+# Import basic module tests, and execute with ipa_context set to 'client'.
+# If ipaclients is set, it will be executed using the client, if not,
+# ipaserver will be used.
+#
+# With this setup, tests can be executed against an IPA client, against
+# an IPA server using "client" context, and ensure that tests are executed
+# in upstream CI.
+
+- name: Test trust using client context, in client host.
+  import_playbook: test_trust.yml
+  when: groups['ipaclients']
+  vars:
+    ipa_test_host: ipaclients
+
+- name: Test trust using client context, in server host.
+  import_playbook: test_trust.yml
+  when: groups['ipaclients'] is not defined or not groups['ipaclients']
+