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/octavia_tempest_plugin/config.py b/octavia_tempest_plugin/config.py
index 7c4ed2f..d951a44 100644
--- a/octavia_tempest_plugin/config.py
+++ b/octavia_tempest_plugin/config.py
@@ -173,7 +173,7 @@
                default='ubuntu',
                help='The amphora SSH user.'),
     cfg.StrOpt('amphora_ssh_key',
-               default='/tmp/octavia_ssh_key',
+               default='/etc/octavia/.ssh/octavia_ssh_key',
                help='The amphora SSH key file.'),
     # Environment specific options
     # These are used to accomodate clouds with specific limitations