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.

This change replaces image_ssh_user with image_alt_ssh_user
and modifies Tempest's config to have access to said
variable. It also adds a password variable in Tempest's
config for the alternative image.

Change-Id: Ibe81a068c6fdeb7cd1eedf1df76ce62737160a01
Closes-Bug: #1844535
Depends-On: https://review.opendev.org/682902/
diff --git a/releasenotes/notes/add-image-alt-ssh-user-config-option-1b775af2f468aa5b.yaml b/releasenotes/notes/add-image-alt-ssh-user-config-option-1b775af2f468aa5b.yaml
new file mode 100644
index 0000000..159bbe8
--- /dev/null
+++ b/releasenotes/notes/add-image-alt-ssh-user-config-option-1b775af2f468aa5b.yaml
@@ -0,0 +1,10 @@
+---
+features:
+  - A new config option in the validation section, image_alt_ssh_user,
+    to specify the user name used to authenticate to an alternative
+    instance (instance using image_ref_alt) in tests. By default this
+    is set to root.
+  - A new config option in the validation section, image_alt_ssh_password,
+    to specify the password used to authenticate to an alternative
+    instance (instance using image_ref_alt) in tests. By default this
+    is set to password.