[CI] Use the script module for running core tests

Depending on the way the testing script is called, it may sense
stdin being available and not being a tty [1]. The logic in the
Glance part of the client is then broken [2] as it detects both
volume and stdin as sources of the image and fails.
The proposed approach uses the script module for these tests.
This fixes the cephadm upgrade jobs where this script was called
using the shell module which causes this behaviour. The script
module is not affected. FWIW, the command module is affected
the same as shell because shell extends the command module.

[1] https://opendev.org/openstack/python-openstackclient/src/commit/6810414e45a32dd44263dff47fec161989508ef0/openstackclient/image/v2/image.py#L114-L120
[2] https://opendev.org/openstack/python-openstackclient/src/commit/6810414e45a32dd44263dff47fec161989508ef0/openstackclient/image/v2/image.py#L414

Change-Id: I3ce4059063e3a51eb1c5b42bda79893b56bd85f0
Story: 2006294