[customize_image] Override infra mirror only if set

Follow up of [1], in local setup it's not needed
as mirrors not set.

[1] https://review.opendev.org/c/openstack/neutron-tempest-plugin/+/954240
Related-Bug: #2110191

Change-Id: Id73d1a2f66a6376dcd4c91ea01524ebd133c44c3
Signed-off-by: yatinkarel <ykarel@redhat.com>
1 file changed