copr-be: better ssh client configuration

- don't wait too long for SSH connection timeout (set it to 60s), we
  we will be restarting the build by backend when the last 4 attempts to
  contact builder failed more quickly than each 2 minutes.
- make the keep alive interval much smaller than the connection timeout
  is

roles/copr/backend/files/ssh_config

@@ -2,4 +2,6 @@ Host *
    IdentityFile ~/.ssh/id_rsa
    StrictHostKeyChecking  no
    UserKnownHostsFile /dev/null
-   ServerAliveInterval 60
+   ServerAliveInterval 20
+   ServerAliveCountMax 5
+   ConnectTimeout 60