Move ARP responder test to sanity command

Move ARP responder test to sanity command

Additionally, the patch improves the check itself:
To check if the currently installed OVS supports the ARP responder
feature, we try to add a flow that references an OpenFlow ARP
extension via ofctl. The test may fail due to an (expected)
Runtime error, or due to some other unexpected error.
In such a case the error was previously masked and tossed away.

* Clean up ARP responder unit test
* Extract ARP responder flow actions to be used by the unit
  tests, functional test as well as the ARP responder code itself

After this patch, if the sanity check returned False but the
user never ran it or ignored its results, the OVS agent will
output errors to the log every time an ARP entry is (attempted)
to be added or removed from the flow table.

Closes-Bug: #1323718
Change-Id: I428c954d6561cd398a1e580804a9482969a154af