Description of problem:
-----------------------
Null values are not handled correctly, which leads to just key being dumped to the inventory file.

E.g: 
/usr/bin/tripleo-ansible-inventory --static-inventory oc-hosts
ansible -i oc-hosts Controllers -m ping -vv
Using /etc/ansible/ansible.cfg as config file
ERROR! Attempted to read "oc-hosts" as YAML: Syntax Error while loading YAML.

The error appears to have been in '/home/stack/oc-hosts': line 1, column 16, but may
be elsewhere in the file depending on the exact syntax problem.

The offending line appears to be:

[aodh_evaluator:vars]
               ^ here

Attempted to read "oc-hosts" as ini file: oc-hosts:366: Expected key=value, got: cacert

Offending line: 
===============
[undercloud:vars]
...
auth_url = http://192.168.24.1:5000/
ansible_connection = local
--->cacert
undercloud_swift_url = http://192.168.24.1:8080/v1/AUTH_f83abc228845445dbc8cea361ee276ff
plan = overcloud

And when passing output through 'python -m json.tool':
======================================================
/usr/bin/tripleo-ansible-inventory --list | python -m json.tool
...
    "undercloud": {
        "hosts": [
            "localhost"
        ],
        "vars": {
            "ansible_connection": "local",
            "auth_url": "http://192.168.24.1:5000/",
            "cacert": null,
            "undercloud_swift_url": "http://192.168.24.1:8080/v1/AUTH_f83abc228845445dbc8cea361ee276ff",
            "username": "admin"
        }
...

Version-Release number of selected component (if applicable):
-------------------------------------------------------------
openstack-tripleo-validations-7.2.1-0.20170807182119.1f60b6f.el7ost.noarch

How reproducible:
-----------------
So far 100%