With switch to keystone::resource::service_identity we lost api version
string from keystone url. We now have CONFIG_KEYSTONE_API_VERSION option
which defaults to v2.0 but can be set to v3.
Change-Id: I6a8ebc18268a06c1eebffc78d9374e907cf92b4f
(cherry picked from commit 2d6e97c009)