Fix references to #mpls_tag{} after deletion of ether_type field

Commit esl/pkt@772ad69a66a4056a8c6ce1032befe654ce01a357 fixed handling
MPLS packets. It removed unnecessary and confusing field ether_type that is NOT
present in MPLS frames.
(Motivated by FlowForwarding/LINC-Switch#144).
Extend action_pop_mpls test by ensuring that ether_type field is set correctly

OpenFlow 1.3.2 clarifies that the ether_type field should be set by the switch
regardless of the BoS bit in the MPLS header.
(Motivated by #144).
Fix applying action_pop_mpls so that ether_type field is set correctly

OpenFlow 1.3.2 clarifies that ehther_type filed indicating MPLS payload
should be set regardless of the BoS bit in the MPLS header.
(Motivated by #144).
Add pcap file with MPLS and controller's scenario for testing pop_mpls action

This scenario along with the pcap.data/mpls.pcap and Wireshark analysis allows
verify that MPLS tag is removed for a matched packet and correct EtherType
is set for the MPLS payload. This verification is dictated by clarification
introduced in OpenFlow 1.3.2
(Motivated by FlowForwarding/LINC-Switch#144).
Revert "In linc_us4_packet:set_field, values are always whole bytes"

This reverts commit 2ba318d620254ebe2cebf3c7c3447ca66c01dab2.

After introducing padding removal into the decoder all match fields'
values (that are also parts of set_field) are stripped to the correct
size. So it's no longer need to extract bits for ECN, DSCP or VLAN PCP
values.
(Motivated by #144).
Fix setting VLAN ID

After introducing stripping binary values of match fields to their
correct sizes linc_us4_packet:set_field/2 had to be fixed for setting
VLAN ID.
(Motivated by #144).
Merge pull request #157 from FlowForwarding/issue144

Fix applying MPLS related actions and add test scenario