Support for imix packet sizes

Support for imix packet sizes

All packet size in the *.test files have been replaces with imix sizes.
imix is now a list of packet sizes. If you want to run with only a
pakcet size of 64, you need to specify a list containing only 1 size:
[64].
You can also specify multiple imix values by specifying a list of list
of sizes. The keyworkds in the test files are now imix & imixs.

Change-Id: Iea47c0266b022133b7bbfc9a6811e3199ae41521
Signed-off-by: Luc Provoost <luc.provoost@intel.com>