DiscoveryFailure exception in plugin setup is misleading

[Colleen Murphy]

During plugin setup we catch DiscoveryFailure, HttpError, and ConnectionError and log a warning:

'Failed to discover available identity versions when contacting %s. Attempting to parse version from URL.' % self.auth_url

http://git.openstack.org/cgit/openstack/keystoneauth/tree/keystoneauth1/identity/generic/base.py?h=3.3.0#n139

This points to a "discovery error", the meaning of which is completely unclear to the average user, and the reality is it could be a connection error related to a firewall rule or keystone not running or keystone running on the wrong port or other things that are easier to debug and understand than "discovery".

We also have this catch-all DiscoveryFailure raised here:

http://git.openstack.org/cgit/openstack/keystoneauth/tree/keystoneauth1/identity/generic/base.py?h=3.3.0#n191

which is probably inappropriate.

We should make these warnings and errors more fine-grained so people understand what to look for when keystoneauth fails.