Fix enum validation in manila shrink and extend scenarios
Fix enum validation in manila shrink and extend scenarios

Missed a comma in https://review.openstack.org/#/c/469170/12/rally/plugins/openstack/scenarios/manila/shares.py

Change-Id: Ie2e586e88573626de74585caf2d48a27e4c259cb

yeming233/rally,yeming233/rally,openstack/rally,openstack/rally,openstack/rally,openstack/rally