Remove argument to ssh-keygen for key size

We don't need to be explicit, ssh-keygen will pick a good size for us.
Meanwhile, 1024 bit keys are generally seen as security problems and
thus ring bells and are refused by some servers/services.

Change-Id: Iaea82e0b394a5a6b1da3b59637fc4e0f541e1978

roles/add-build-sshkey/tasks/create-key-and-replace.yaml

@@ -1,5 +1,5 @@
 - name: Create Temp SSH key
-  command: ssh-keygen -t rsa -b 1024 -N '' -C 'zuul-build-sshkey' -f {{ zuul_temp_ssh_key }}
+  command: ssh-keygen -t rsa -N '' -C 'zuul-build-sshkey' -f {{ zuul_temp_ssh_key }}
   delegate_to: localhost
   run_once: true