fix(nat44det): reduce ramp-up rate

The previous values were too high for 9000b bandwidth.
In order to avoid any packet loss,
the new values are low enough even for x710 (10 Gbps).

The ramp-up rate is also considerably lowered
for lower scales, to keep the rate constant.
Ramp-up duration slightly increased above the minimum needed
to avoid time overheads, duration stretching and rounding errors.

Ticket: CSIT-1811

Change-Id: Iefa8d4d9344e3876014dc5415a4acfedc5abf429
Signed-off-by: Vratko Polak <vrpolak@cisco.com>