Description of problem:
OSP 13 supports subnet configuration on undercloud, but the validation playbook only check the default sections


https://access.redhat.com/documentation/en-us/red_hat_openstack_platform/13/html/director_installation_and_usage/installing-the-undercloud#director-configuration-parameters_subnets
https://github.com/openstack/tripleo-validations/blob/stable/queens/validations/ctlplane-ip-range.yaml

How reproducible:

Constantly 

Steps to Reproduce:
1. Added a subnet configuration to the overcloud config
2. run the validation playbook

Actual results:
[stack@undercloud-0 ~]$ ansible-playbook /usr/share/openstack-tripleo-validations/validations/ctlplane-ip-range.yaml  -i testinv --user stack

PLAY [undercloud] *************************************************************************************************************************************************************************************************

TASK [Gathering Facts] ********************************************************************************************************************************************************************************************
ok: [undercloud-0]

TASK [Get the path of tripleo undercloud config file] *************************************************************************************************************************************************************
ok: [undercloud-0]

TASK [Gather undercloud.conf values] ******************************************************************************************************************************************************************************
ok: [undercloud-0]

TASK [Check the size of the DHCP range for overcloud nodes] *******************************************************************************************************************************************************
fatal: [undercloud-0]: FAILED! => {"changed": false, "msg": "The IP range 192.0.2.5 - 192.0.2.24 contains 20 addresses.\nThis might not be enough for the deployment or later scaling."}
 [WARNING]: Could not create retry file '/usr/share/openstack-tripleo-validations/validations/ctlplane-ip-range.retry'.         [Errno 13] Permission denied: u'/usr/share/openstack-tripleo-
validations/validations/ctlplane-ip-range.retry'


PLAY RECAP ********************************************************************************************************************************************************************************************************
undercloud-0               : ok=3    changed=0    unreachable=0    failed=1   

Expected results:

[stack@undercloud-0 ~]$ ansible-playbook /usr/share/openstack-tripleo-validations/validations/ctlplane-ip-range.yaml  -i testinv --user stack

PLAY [undercloud] *************************************************************************************************************************************************************************************************

TASK [Gathering Facts] ********************************************************************************************************************************************************************************************
ok: [undercloud-0]

TASK [Get the path of tripleo undercloud config file] *************************************************************************************************************************************************************
ok: [undercloud-0]

TASK [Gather undercloud.conf values] ******************************************************************************************************************************************************************************
ok: [undercloud-0]

TASK [Check the size of the DHCP range for overcloud nodes] *******************************************************************************************************************************************************
ok: [undercloud-0]

PLAY RECAP ********************************************************************************************************************************************************************************************************
undercloud-0               : ok=4    changed=0    unreachable=0    failed=0

Additional info:

This works in OSP 14 
https://github.com/openstack/tripleo-validations/blob/stable/rocky/validations/ctlplane-ip-range.yaml