test: log SSH instructions

[r2k1]

/kind test

Log instruction on how to easy SSH into VM.

Example:

...
    exec.go:199: SSH Instructions: (VM will be automatically deleted after the test finishes, set KEEP_VMSS=true to preserve it or pause the test with a breakpoint before the test finishes)
        ========================
        az account set --subscription 8ecadfc9-d1a3-4ea4-b844-0d9f87e4d7c8
        az aks get-credentials --resource-group abe2e-westus3 --name abe2e-kubenet-331fc --overwrite-existing
        kubectl exec -it debug-mariner-gxh2f -- bash -c "chroot /proc/1/root /bin/bash -c 'ssh -i sshkey10224010 -o PasswordAuthentication=no -o UserKnownHostsFile=/dev/null -o StrictHostKeyChecking=no -o ConnectTimeout=5 [email protected]'"
...