ansible-freeipa.spec: Bump minimum supported Ansible version to 2.15

ansible-freeipa roles do not work with Ansible 2.8 anymore, so the
minimum supported version is changed to 2.15, the oldest supported
Ansible version as of today.

This patch updates the minimum supported Ansible version and the list
of available modules.

utils/ansible-freeipa.spec.in

@@ -90,7 +90,7 @@ Supported Distributions
 Requirements
 
   Controller
-  - Ansible version: 2.13+
+  - Ansible version: 2.15+
   - /usr/bin/kinit is required on the controller if a one time password (OTP)
     is used