Remove usage of $[ for arithmetic

$[ is deprecated for the more familiar $((

Required when bash8 starts testing from tox and [1] goes in

[1] https://review.openstack.org/#/c/117430/

Change-Id: I8ba180be036836f37ebdbb6da36ff0be486c043e
Merge "Remove usage of $[ for arithmetic"
Merge branch 'master' of https://github.com/openstack-dev/devstack