cluster: network: clarify and ease latency requirements

cluster: network: clarify and ease latency requirements

With 5 ms (stable!) even mid size cluster can work just fine, and we
know quite some smaller setups that work OK with 6 - 9 ms, so ease
on unnecessarily scaring users into thinking that their network will
cause them trouble.

we got reports of working two node clusters with ~15 ms latency, so
it can work out above 10ms, albeit things start to get really brittle
and def. not something for 3+ nodes.

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>