enable enforcing of bash8 in the devstack gate

this ensures that our style checker gets voting authority on our
patches, hopefully reducing the number of style issues we need to
-1 in the gate manually.

It requires this patch series to have landed first:
https://review.openstack.org/#/q/branch:stable/havana+topic:bash8_stable,n,z

Change-Id: Id6497be682a961e3a9d648d8d0187bcffa3e70fb
Merge "enable enforcing of bash8 in the devstack gate"