The current auth plugin system lacks some functionality to be used with
other methods that might require additional configuration options or
that do not require a user to pass some options that are now compulsory
(for example, X.509 authentication needs to get a certificate file, and
does not need either a username or a password). This commit extends
the current system to handle these extra features, while remaining
compatible with older plugins.
DocImpact: We should documment how to implement additional
authentication plugins, such as BasicAuth, X509, etc.
Implements: blueprint authentication-plugins
Change-Id: I7b0ef4981efba8160dea94bf852dba7e2e4068f5
abd75f24b1