pkt_gen: Add IxNet support for GRE frames

pkt_gen: Add IxNet support for GRE frames

Add option to generate GRE frames for testing GRE decapsulation
tests.

Recent changes:
* Fix merge conflict
* Updated userguides
* Complete set of fields and working decap
* Move GRE ixnet calls to if statement to reuse inner frame settings

Change-Id: I0cf9243953a72a464ae4701a858e76aecbdb8528
JIRA: VSPERF-191
Signed-off-by: Dino Simeon Madarang <dino.simeonx.madarang@intel.com>
Reviewed-by: Maryam Tahhan <maryam.tahhan@intel.com>
Reviewed-by: Billy O Mahony <billy.o.mahony@intel.com>
Reviewed-by: Martin Klozik <martinx.klozik@intel.com>
Reviewed-by: Al Morton <acmorton@att.com>
Reviewed-by: Brian Castelli <brian.castelli@spirent.com>