Add extension model for supporting SPAN/ERSPAN
Add extension model for supporting SPAN/ERSPAN

Change-Id: I145e5da3a4c227aa6289542bf6e76479ae4de88a
Signed-off-by: Tom Flynn <abc7c57a6537c493534eb3847500f37bef7b716d@gmail.com>

opendaylight/opflex,mhrobinson/opflex,mhrobinson/opflex,mhrobinson/opflex,mhrobinson/opflex,opendaylight/opflex,opendaylight/opflex,opendaylight/opflex