Allows users to specify a key name in a given project’s `ansible.cfg`
file and thus handle keyring integration with vaults with different
passwords. If no key name is specified, the original default `ansible`
key name will be used.
Other improvements:
* `username` is now optional; defaults to user that invokes the script
* change string interpolation to new `.format()` style
* clean up and expand upon documentation
* enforce PEP 8 compliance
vault-keyring.py was using an older version of
the ansible.constants.load_config_file() API.
The newer version returns a tuple, which caused
the config load to fail and a catch all exception
to blame it on a missing section.
Update to new API, and catch the ConfigParser error
specifically.
Fixes#15984
