Add ICMP probes

These probes check if we can reach several targets via ICMP, which is
not proxied, so they can be used to see if ICMP forwarding is working in
the login node.

In particular, we test if we can reach the Google (8.8.8.8) and
Cloudflare (1.1.1.1) DNS servers, the BSC gateway which responds to ping
only from the intranet and the login node (ssfhead).

Reviewed-By: Aleix Roca Nonell <aleix.rocanonell@bsc.es>