For ERSPAN encap, both bits in the EN field of the header should
be set to indicate any VLAN tag in the original Ethernet frame is
preserved.
Added SPAN L2 test case where the mirrored packet output is a GRE
ERSPAN tunnel.
Change-Id: Ie7a40992a9278469c24aa6fa9e122b4505797d10
Signed-off-by: John Lo <loj@cisco.com>