g5k: using Enoslib through SSH proxy fails when configuring many nodes

When using Enoslib from outside Grid'5000, we need to use the SSH proxy at access.grid5000.fr. However, this fails when trying to configure many nodes. With 14 nodes, Enoslib fails on one of the node at random with this error:

2022-08-30 18:13:50,369 WARNING: terminated: <SshProcess('cat ~/.ssh/id_rsa.pub ~/.ssh/authorized_keys|sudo-g5k tee -a /root/.ssh/authorized_keys', Host('uvb-6.sophia.grid5000.fr'), connection_params={'user': 'user', 'keyfile': '/home/user/.ssh/id_rsa'}, name=cat ~/.ssh/id_rsa.pub ~/.ssh/authori..., started=True, start_date=2022-08-30 18:13:50+02:00, ended=True, end_date=2022-08-30 18:13:50+02:00, killed=False, error=False, error_reason=None, timeouted=False, expect_fail=False, write_error=False, exit_code=65280, ok=False, pid=31818, real cmd=('ssh', '-tt', '-o', 'BatchMode=yes', '-o', 'PasswordAuthentication=no', '-o', 'StrictHostKeyChecking=no', '-o', 'UserKnownHostsFile=/dev/null', '-o', 'ConnectTimeout=20', '-o', 'User=user', '-i', '/home/user/.ssh/id_rsa', 'uvb-6.sophia.grid5000.fr', 'cat ~/.ssh/id_rsa.pub ~/.ssh/authorized_keys|sudo-g5k tee -a /root/.ssh/authorized_keys'))>
stdout:

stderr:
kex_exchange_identification: read: Connection reset by peer
kex_exchange_identification: Connection closed by remote host

After some analysis, it turns out that Enoslib (through Ansible) creates simultaneous SSH connections for all nodes, and each of these connection creates a separate SSH connection to access.grid5000.fr. When there are too many, some of these connection attempts fail.