Add irrelevant-files to nova-multiattach job in .zuul.yaml

This patch set adds irrelevant-files to nova-multiattach job
in .zuul.yaml because it is the only job without such a stanza
which is important for eliminating unnecessary Zuul resource
consumption, which, for example, is happening here:

https://review.openstack.org/#/c/583451/1

Change-Id: Ib48b735d65d2c005197edef9ab3011f93d0ad28c
1 file changed