Add env var for custom ssh user

W/o an env var, a dynamic inventory can't be invoked for a custom
user. Ansible uses the same ANSIBLE_SSH_USER so the value will
be aligned to both.

Related-bug: #1734298

Change-Id: Ieddbb4c87d88888f78d494ff670db907bce4fd78
Signed-off-by: Bogdan Dobrelya <bdobreli@redhat.com>

releasenotes/notes/configurable-ssh-user-840a9ef5416675e9.yaml

@@ -3,3 +3,5 @@ fixes:
   - The ansible_ssh_user in the generated inventory by
     tripleo-ansible-inventory is now configurable instead
     of hardcoded to heat-admin. The default is still heat-admin.
+    Use the `--ansible_ssh_user` param or the `ANSIBLE_SSH_USER`
+    env var to define a custom value.

scripts/tripleo-ansible-inventory

@@ -47,7 +47,8 @@ opts = [
         'OS_PROJECT_NAME', os.environ.get('OS_TENANT_NAME'))),
     cfg.StrOpt('cacert', default=os.environ.get('OS_CACERT')),
     cfg.StrOpt('plan', default=os.environ.get('TRIPLEO_PLAN_NAME')),
-    cfg.StrOpt('ansible_ssh_user', default='heat-admin'),
+    cfg.StrOpt('ansible_ssh_user', default=os.environ.get('ANSIBLE_SSH_USER',
+        'heat-admin')),
 ]