add bash8 to devstack tests

this creates a non-voting bash8 test for devstack which does basic
style checking of the devstack shell scripts.

Requires https://review.openstack.org/#/c/51711/ to land first for
this to work.

Change-Id: Ib6ae29cf4058263aedd3d9c0a1e15de27eea729b
Merge "add bash8 to devstack tests"