Adds information about using bash to documentation.
Adds information about using bash to documentation.

Adds
**Note:** Make sure to use bash, as other shells are not fully compatible
and may cause hard to debug problems.
to the doc/source/devref/devstack.rst to warn that bash should be used.

Change-Id: I70f73f26ce8bc75e46cd858c4b33a6a7aff3cdce
Closes-Bug: 1674779

openstack/sahara,openstack/sahara