Make gen_pki.sh & debug_helper.sh bash8 compliant

Now that bash8 is available on PyPI we can use it to clean up the bash
scripts in Keystone. This also uses bash8 in tox. For now we can add
files to the tox check manually as we make them compliant.

Change-Id: I87a7478949114163f0614b1a6d8b249e14afe0df
Merge "Make gen_pki.sh & debug_helper.sh bash8 compliant"