Extract the authentication code from a v2 client and move it to a series
of auth plugins. Auth plugins each represent one method of
authenticating with a server and there is a factory method on the base
class to select the appropriate plugin from a group of arguments.
When a v2 client wants to do authentication it will create
a new v2 auth plugin, do the authentication and then take that result
for the client to use.
Change-Id: I4dd7474643ed5c2a3204ea2ec56029f926010c2c
blueprint: auth-plugins
96b8e81b1d