Add kdump retry/delay mechanism when dumping over network

Kdump currently try mounting NFS (or doing the SSH dump) only once, and
if it fails, it just gives-up. Since kdump may be essential to debug hard
to reproduce bugs, we should improve the resilience and retry a bit,
delaying at each attempt.

This patch introduces a retry/delay mechanism for both NFS and SSH dumps;
the delay time is the same but number of retries is different (since NFS
mounts takes a long time between failures and is inherently more resilient),
both being configurable parameters from /etc.

The original trigger of this issue is a long-term (bad) behavior of some
NICs, which present a "Link Up" status _before_ being ready to transmit
packets; hence network kdump will try and fail without this patch.

Signed-off-by: Guilherme G. Piccoli <gpiccoli@canonical.com>
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@debian.org>