Don't set priority when calling mod_flow

This commit [1] changed the behavior of modifying flows to not allow priority
to be set on flow modification. The agent code which was calling mod_flow was
not updated to respect this behavior. The unit tests were also not updated in
this regard. I've corrected this behavior now, and added a negative UT case to
catch this in the future.

[1] https://review.openstack.org/#/c/58533/

Closes-Bug: #1295703

Change-Id: I20737637cc84567a5ec19efaad18e7f334007f18
Merge "Don't set priority when calling mod_flow"