Fix RBD store to use READ_CHUNKSIZE
Fix RBD store to use READ_CHUNKSIZE

Change 4e0c563b8f3a5ced8f65fcca83d341a97729a5d4 was incomplete and
missed a variable in the rbd store.

This patch fixes the issue by using store.READ_CHUNKSIZE instead of
store.chunk_size

Change-Id: I2dc5b1514d3e86ec129c1fc88de096d98a64f806
Closes-Bug: #1352509

takeshineshiro/glance,klmitch/glance,dims/glance,jumpstarter-io/glance,paramite/glance,stevelle/glance,openstack/glance,kfwang/Glance-OVA-OVF,openstack/glance,vuntz/glance,klmitch/glance,saeki-masaki/glance,kfwang/Glance-OVA-OVF,wkoathp/glance,saeki-masaki/glance,jumpstarter-io/glance,scripnichenko/glance,openstack/glance,darren-wang/gl,wkoathp/glance,dims/glance,redhat-openstack/glance,scripnichenko/glance,takeshineshiro/glance,sigmavirus24/glance,darren-wang/gl,stevelle/glance,ozamiatin/glance,vuntz/glance,ozamiatin/glance,rajalokan/glance,akash1808/glance,JioCloud/glance,rajalokan/glance,JioCloud/glance,sigmavirus24/glance,redhat-openstack/glance,paramite/glance,akash1808/glance