ansible_tower: Add custom_virtualenv attribute when applicable (#60200)

In Ansible Tower/AWX, there are three kinds of objects that can be tied
to custom python virtual environment:
  - job template
  - project
  - organization

This patch updates the three ansible modules that creates those objects
so that the 'custom_virtualenv' attribute can be set if specified.

Testing Done: via a playbook, test organization, projet then template creation
without any 'custom_virtualenv' attribute specified. Check that the
resources get created and that their python env is set to default. Then
re-do the same test but this time with the 'custom_virtualenv' attribute
specified. Ensure in AWX UI that those resources have the right
'custom_virtualenv' set.

awx_collection/plugins/modules/tower_project.py

@@ -87,6 +87,8 @@ options:
       description:
         - Local absolute file path containing a custom Python virtualenv to use
       type: str
+      required: False
+      default: ''
     organization:
       description:
         - Primary key of organization for project.
@@ -156,7 +158,7 @@ def main():
         scm_update_on_launch=dict(type='bool', default=False),
         scm_update_cache_timeout=dict(type='int'),
         job_timeout=dict(type='int', default=0),
-        custom_virtualenv=dict(),
+        custom_virtualenv=dict(type='str', required=False, default=''),
         local_path=dict(),
         state=dict(choices=['present', 'absent'], default='present'),
         wait=dict(type='bool', default=True),