app/testpmd: disable Rx VLAN offloads by default

app/testpmd: disable Rx VLAN offloads by default

Removed the hardcoded preconfigured Rx VLAN offload configuration
from testpmd and changed the Rx offload command line parameters from
disable to enable.

It has been decided by the Technical Board that testers who wish to
use these offloads will now have to explicitly write them in the
command-line when running testpmd.
The agreement is to keep two exceptions enabled by default in 18.02:
Rx CRC strip and Tx fast free.

Motivation:
Some PMDs such at the mlx4 may not implement all the offloads.
After the offload API rework assuming no offload is enabled by default,
  commit ce17eddefc20 ("ethdev: introduce Rx queue offloads API")
  commit cba7f53b717d ("ethdev: introduce Tx queue offloads API") trying
to enable a not supported offload is clearly an error which will cause
configuration failing.

Considering that testpmd is an application to test the PMD, it should
not fail on a configuration which was not explicitly requested.
The behavior of this test application is then turned to an opt-in
model.

Signed-off-by: Moti Haimovsky <motih@mellanox.com>
Acked-by: Thomas Monjalon <thomas@monjalon.net>