[Varnish] 4.x health command

With previous varnish version, the “undocumented-command-to-know” to check backend health was:

varnishadm debug.health

A return code 200 was great and any other value bad. Problem: this command basically give no details on the probe setting and it success rate.

With version 4.x varnish the command has changed. Now it’s:

varnishadm backend.list

and this time it not only give you a list of each backend but also the associated probe success rate.