testing: Explicitly enable RC4 in SSH server config

Newer OpenSSH versions disable this by default because it's unsafe.
Since this is not relevant for our use case we enable it due to its
speed.

testing/hosts/default/etc/ssh/sshd_config

@@ -1,5 +1,6 @@
 Port 22
 Protocol 2
+Ciphers arcfour
 HostKey /etc/ssh/ssh_host_rsa_key
 HostKey /etc/ssh/ssh_host_dsa_key
 HostKey /etc/ssh/ssh_host_ecdsa_key