Increase number of devices for aluminium NETCONF scale test

Increase number of devices for aluminium NETCONF scale test

Do not waste cycles on small device count, let assume we have
1hr. max per run, then we can fit 11-12 runs into 12hr budget,
starting at 1k and increment by 500 and stop at 4k devices
should give us enought time to complete 7 test runs (1k, 1.5k,
2k, 2.5k, 3k, 3.5k, 4k).

Currently we're spending approximately tens of minutes per run:
2200 .. 09:49:35.932 - 10:06:42.953 ~ 17 min.
2400 .. 10:58:29.420 - 11:16:41.775 ~ 18 min.
2600 .. 12:28:04.384 - 13:49:35.439 ~ 22min.
2800 .. 14:15:24.558 - 14:46:23.139 ~ 31 min.

Change-Id: Iee9c46447440eef8f78e55abc36be5e3d839fc6d
Signed-off-by: Miroslav Miklus <miroslav.miklus@pantheon.tech>