Add scenario for changing controller's roles

This scenario helps to verify that the switch responds with the correct
generation id replying to a role request message.
Fix linc_us4_packet:set_field for DSCP and ECN fields of IPv6

The class field of the #ipv6 record is an integer, but this function was
treating it as a binary.
Use correct types in linc_us4_packet_tests

The value field in an #ofp_field record must always be a binary.
Updated tests accordingly.  Also changed initial and expected values to
be binaries where appropriate.

Also make the corresponding change in linc_us4_actions_tests.
Test that packets can be encoded before and after edit

Call the corresponding pkt functions from linc_us4_packet_tests.
Merge pull request #152 from legoscia/fix-packet-edit-tests

Fix packet edit tests
Merge pull request #151 from FlowForwarding/issue143

Add scenario for changing controller's roles
Scenario for #153: set IPv4 ECN field

This doesn't seem to work; the ECN field doesn't get set.