barbican: Use python3 plugin in uwsgi config

backport: ussuri, train

Without this the container returns an empty response.

Change-Id: Ic36845f3fc625c080c92904b58ace070dd24fbb2
Closes-Bug: #1881784

ansible/roles/barbican/templates/barbican-api.ini.j2

@@ -9,6 +9,6 @@ lazy = true
 vacuum = true
 no-default-app = true
 memory-report = true
-plugins = python
+plugins = python3
 paste = config:/etc/barbican/barbican-api-paste.ini
 add-header = Connection: close