Enabling Openflow NXM Extensions Karaf Feature

1) Feature Tests Pass:
https://jenkins.opendaylight.org/ovsdb/job/ovsdb-merge/257/org.opendaylight.ovsdb$features-openflow-nxm/testReport/

2) Features Comply with Guidelines: Yes (features starts with odl-, with description & no Start-levels)
https://git.opendaylight.org/gerrit/gitweb?p=ovsdb.git;a=blob;f=features/of-nxm-extensions/src/main/resources/features.xml;h=fe25b554230729f1aee0b46b38246db1734a4aff;hb=c46bca34b1e903c71a485b4b5255a4996b2f09e4

3) Features Tested: Features have been tested for functionality with
   feature:install odl-openflow-nxm-extensions

4) Integration Test Impact : This brings in the NXM extensions support for Openflow13. Should not have any impact.

Signed-off-by: Madhu Venugopal <mavenugo@gmail.com>