Description of problem:

Running 'ctlplane-ip-range' pre-introspection check against undercloud.conf with default values for ctlplane-subnet returns misleading error

Version-Release number of selected component (if applicable):

13-z

How reproducible:

100%

Steps to Reproduce:
1. Setup undercloud.conf with default ctlplane-subnet values for dhcp_start, dhcp_end
2. Run ctlplane-ip-range validation

Actual results:

[...]
TASK [Check the size of the DHCP range for overcloud nodes] ********************
fatal: [undercloud]: FAILED! => {"changed": false, "msg": "Argument start () must be an IP\nArgument end () must be an IP"}

PLAY RECAP *********************************************************************
undercloud                 : ok=4    changed=0    unreachable=0    failed=1   

Expected results:


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

PLAY RECAP *********************************************************************
undercloud                 : ok=5    changed=0    unreachable=0    failed=0   

Additional info:

Workaround:
1) edit undercloud.conf
a) for each control plane subnet (default = ctlplane-subnet) section:
i) Specify a value for dhcp_start (e.g. "dhcp_start = 192.168.24.5")
ii) Specify a value for dhcp_end (e.g. "dhcp_end = 192.168.24.29")
Rerun validation

N.B. The check is for >= 25 ip addresses in range (see https://docs.openstack.org/tripleo-validations/queens/validations-pre-introspection-details.html#ctlplane-ip-range).  This will be addressed in a separate bz