Update c9s job with new ssh key type

RSA ssh keys are now rejected by ssh client in c9s, generate a ECDSA key
for this job.

Change-Id: Ia8b90d74b657612a02bc2c30514f09f179ab97e3

zuul.d/jobs.yaml

@@ -725,6 +725,13 @@
         OCTAVIA_AMP_BASE_OS: centos
         OCTAVIA_AMP_DISTRIBUTION_RELEASE_ID: 9-stream
         OCTAVIA_AMP_IMAGE_SIZE: 3
+        OCTAVIA_SSH_KEY_TYPE: ecdsa
+        OCTAVIA_SSH_KEY_BITS: 256
+      devstack_local_conf:
+        test-config:
+          "$TEMPEST_CONFIG":
+            validation:
+              ssh_key_type: ecdsa
 
 - job:
     name: octavia-v2-dsvm-scenario-ubuntu-focal