Increase the default number of rx and tx descriptors to 2K

Increase the default number of rx and tx descriptors to 2K

A high number of RX descriptor can be helpful in avoiding
packet loss due to the core being interrupted.
This commit increases the default number of descriptors from 256 to 2K.

Those values are checked versus the minimum and maximum supported by the pmd.
Number of mbufs (mempool size) have been updated n config files to support this
default number of dsescriptors.

The number of descriptors can still be overwritten by the config file. However
those values must always remain within the limits reported by the pmd.

Change-Id: I5f7999eca886dad68f3c0733da0d796bf4c06a56
Signed-off-by: Xavier Simonart <xavier.simonart@intel.com>