Ensure ofagent unit tests handles random hashseeds

Several tests in test_ofa_neutron_agent.py and test_ofa_flows.py fail when
running the unit tests with hashseed 2701526934.

This patch fixes the failing unit tests by specifying that assert_has_calls does
not need to assert any order.

Change-Id: I48d6601130f7de1f6eeb7333a17eddd82e7acbdf
Partial-Bug: 1348818
Remove duplicate import of constants module

.. and enable corresponding pylint check now the only offending instance
is fixed.

Change-Id: I35a12ace46c872446b8c87d0aacce45e94d71bae
Merge "Ensure ofagent unit tests handles random hashseeds"
Merge "Remove duplicate import of constants module"