Use PUSH_VLAN action to add VLAN tag to untagged frame.

This patch is required to support OF 1.3 switch because SET_FIELD
action never appends a VLAN tag to the packet. OF plugin will ignore
PUSH_VLAN action if OF 1.0 is used.

Change-Id: If194a4629a7f8b70acc0dc276b1899b48c2e382e
Signed-off-by: Shigeru Yasuda <s-yasuda@da.jp.nec.com>