Add bash8 to global-requirements

Now that bash8 is available on PyPI, we can start using it on OpenStack
projects. Looking to start using bash8 in Keystone bash scripts. Note
that bash8 uses the Apache Software License 2.0.

Change-Id: Ica767630f68f60dc36dba8fd5bf0bce74342b2f2
Merge "Add bash8 to global-requirements"