Add retries to iSCSI connect_volume
Add retries to iSCSI connect_volume

It's possible for Nova and Cinder to both call in to os-brick
to have a connect and disconnect overlap. These methods have
locking around them to prevent clashes, but in this case each
service's locks could be in a separate directory and still end
up clashing.

Since initial volume connect does not have IO running yet, we
can just handle the case where the connection gets yanked out
from under us by retrying the connect again.

Change-Id: Ic14e442c656b81c17a983166b27c996fa1aa8b83

openstack/os-brick,openstack/os-brick