The kolla-genpwd and kolla-mergepwd commands can be used to manipulate
the kolla passwords.yml file. The format is a YAML encoded dict of
password variable names to their values. If the format is not a dict,
the error messages are unhelpful.  In particular, this can happen if the
file is encrypted e.g. via Ansible Vault.

For kolla-genpwd:

    AttributeError: 'NoneType' object has no attribute 'items'

For kolla-mergepwd:

    AttributeError: 'NoneType' object has no attribute 'update'

This change adds a more friendly message.

Change-Id: I27f0835b904e05006ae401adf383090322e1b891
Closes-Bug: #1880220