openstack: Document the python-novaclient requirement (#40842)

Without also installing the python-novaclient library, these modules
simply do not work.

Rather than backporting the fixes implemented in 2.6+ we simply make
sure that we document the requirement.

Fixes: #35484

lib/ansible/modules/cloud/openstack/os_server.py

@@ -199,6 +199,7 @@ options:
      required: false
 requirements:
     - "python >= 2.6"
+    - "python-novaclient"
     - "shade"
 '''
 

lib/ansible/modules/cloud/openstack/os_server_action.py

@@ -66,6 +66,7 @@ options:
      required: false
 requirements:
     - "python >= 2.6"
+    - "python-novaclient"
     - "shade"
 '''