hg_ping command for testing connectivity

[knard38]

To efficiently diving into networking issues an hg_ping command is missing.
The goal of such command will be to check connectivity, data transfert, etc. such as ibping, rping, ucmatose or ibv_rc_pingpong.

A workaround to this missing command is to use the hg_rate in the following way:

1. Start an hg_perf_server at the destination node.
2. Run the following command from the client node

timeout --kill=3s 30s bash -c 'hg_rate -f ./port.cfg -c ucx -p dc_mlx5 -d ib0:1 -H 192.168.100.1 -V &> /tmp/mercury-client.log'

The value return by this last command can then be used to check if the destination node is reachable with the mercury library.

NOTE: It is not possible to directly use the hg_rate command as it retries indefinitely to reach the server when this last one is not reachable.