Use 'password' auth_type by default

Use 'password' auth_type by default

It's better to use 'password' keystone auth plugin by default
for the supporting v2 and v3 keystone API instead of ceilometer
legacy keystone loader which supports only v2.
Remove hard-coded API version from auth_url as well.

Actually this legacy loader has been deprecated in Mitaka and
has been already removed in Newton:
https://review.openstack.org/#/c/307053/

Change-Id: Iad68b6c1c29f9d9cd1b41ecc2898732fa030d401