Do not generate RSA key for runner sshd

[r4victor]

Part of #3920

Drop RSA host key generation for runner's sshd on every container start. RSA keys are very slow to generate (~1s). This leads to runner API being unavailable for some time after the container start. The dstack server fails to connect to connect to the runner on the first attempt, and provisioning may require extra processing cycles. ECDSA and Ed25519 keys should be sufficient as Ed25519 is supported since OpenSSH 6.5 (2014) and ECDSA since 5.7 (2011).

# time ssh-keygen -t rsa -b 3072 -q -N "" -f /tmp/rsa_key

real	0m0.894s
user	0m0.892s
sys	0m0.001s

# time {
   ssh-keygen -t ecdsa   -q -N "" -f /tmp/test/ssh_host_ecdsa_key
   ssh-keygen -t ed25519 -q -N "" -f /tmp/test/ssh_host_ed25519_key
}

real	0m0.010s
user	0m0.008s
sys	0m0.002s