Added backend.list Command

[viable-hartman]

Added backend.list command to Varnish 4 Commands. This command allows you to list your configured Varnish backends similar to below:

Backend name Admin Probe boot.web01 probe Healthy 6/6 boot.web02 probe Healthy 6/6 boot.web03 probe Sick 2/6

My code returns this output in an associative array where the KEY names are row 1 from above, and point to arrays that hold the respective values below the key.