Set image_alt_ssh_user during stack

At this moment, only image_ssh_user is present in the config
of Tempest. It's set to cirros by default and used for
SSH connections in tests. However, several tests build
instances with image_ref_alt, but still use image_ssh_user to
connect, which results in failure if image_ref_alt is set to
a non-cirros image. They should use image_alt_ssh_user instead,
which can be set to whichever user the image_ref_alt needs in
either local.conf or during plugin installation.

Change-Id: I899909fb71a9862c891e94ba54c6a8fa137f9769
Partial-Bug: #1844535
1 file changed