configure sshd with ClientAliveInterval

Pushing large repositories fails with:

==> /var/log/gitlab-shell/ssh.log <==
Received disconnect from 10.244.9.113 port 35550:11: disconnected by user
Disconnected from 10.244.9.113 port 35550

client side message is:

 fatal: The remote end hung up unexpectedly

I've worked around this by specifying a ServerAliveInterval on the client side.

Thank you