(#20677 Raise error on missing path for auth stanza)

This patch raises a ConfigurationError when the user has specified an
allow/allow_ip directive without a preceding path directive.
This is, presumably, a bit more user friendly than having a NilClass
exception thrown when you forget to specify the path directive in your
auth.conf
(#20677) Add tests to ensure configuration errors are raised as expected

I also took the liberty of adding a add_raw_stanza helper function. It
doesn't really do much, but it seems like a good idea to me to keep the specs
as clean as possible. Maybe having two separate cases for allow &
allow_ip is a bit overzealous, but I guess the overhead is minimal.
Merge branch 'pull-1641'

This closes GH-1641