Add hacking rules for shell scripts

This is an attempt to collect the rules that we live by in devstack
that are generally held. Writing these down help us figure out ways
to put them into bash8 over time. These are a starting point for
conversation.

Change-Id: Id2b750665871ebbeddf4694ba080c75d2f6f443e
Merge "Add hacking rules for shell scripts"
Merge branch 'master' of github.com:openstack-dev/devstack