Fix positional formatting and add skip check

This patch fixes an offset positonal argument in a string format that
was throwing an IndexError exception. It also adds a skip check to check
if the required amphora SSH key file is readable.

Additionally, the iptables-based active-standby test has now a better
amphora_ssh_key default value to match with the default path set by the
Octavia Devstack plugin.

Story: 2006259
Task: 35876

Change-Id: Ic887c726dfe151ea582a1bd935ae0b30582f14ea
diff --git a/zuul.d/jobs.yaml b/zuul.d/jobs.yaml
index 42643ed..aa204fd 100644
--- a/zuul.d/jobs.yaml
+++ b/zuul.d/jobs.yaml
@@ -561,6 +561,7 @@
             load_balancer:
               check_timeout: 180
               loadbalancer_topology: 'ACTIVE_STANDBY'
+              amphora_ssh_key: '/tmp/octavia_ssh_key'
       tempest_test_regex: ^octavia_tempest_plugin.tests.act_stdby_scenario.v2.test_active_standby_iptables
       tox_envlist: all