This is next step patch from https://review.openstack.org/#/c/573118/
which makes all the volume service clients alias with _latest suffix.
As all the volume clients point to v3 service client only, having
alias with _v2_ or _v3_ name are confusing.
This commit provide alias with _latest suffix and use them in
tempest. As old alias are being used in tempest plugins also,
we keep the old alias also and slowly will move each plugins to
start using new alias.
Change-Id: Ib89ab72ab4ada3e8442901f104b0b96072187ef9
6c682ffc5d