Update rally-openstack to support block-storage

Update rally-openstack to support block-storage

Block-storage is the official service type for cinder, according to the
service-types-authority starting from OpenStack Stein release [1].

It is unversioned endpoint which replaces old 'volume', 'volumev2' and
'volumev3'.

It updates rally-openstack to the right commit id [2] till the new
rally-openstack release.

[1] https://review.openstack.org/#/c/510939/
[2] https://review.openstack.org/#/c/652405/

Change-Id: I3bdb48d20a55ec6f86ea3c6ccfcbda86057e91a1
Signed-off-by: Cédric Ollivier <cedric.ollivier@orange.com>