Do not run "ovs-ofctl add-flow" with an invalid in_port

Since ofport is a string, we have to convert it to integer to
check it expresses a negative number or not.

Also unit tests are added/corrected.

Change-Id: If85595f1b8ecb40ea41edda52706fb1bd41cb1ab
Related-Bug: 1238445
Merge "Do not run "ovs-ofctl add-flow" with an invalid in_port"