This patch migrates designateclient to using keystoneauth Session
instead of deprecated keystoneclient's Session and plugins.
Also, this patch removes the old designateclient.auth module as it is
not used anywhere else in the code base and its purpose is overridden by
the fact that keystoneauth acts as an abstraction layer for
authentication (handling requests and reauthentication internally).
Change-Id: Ic3cc0fbb76977a04bab322ebcb1c818e75646772
b68dca09b1