Force order of dhcp.needs_resync_reasons dictionary elements

Force order of dhcp.needs_resync_reasons dictionary elements

This fixes the test_periodoc_resync_helper unit test that breaks with
a randomized PYTHONHASHSEED (see the bug report).

The test assumed that the dhcp.needs_resync_reasons dictionary from
neutron.agent.dhcp.agent had elements in a particular order. Found with
PYTHONHASHSEED=2.

The fix refactors the test case to force a sorted dhcp.needs_resync_reasons
dictionary.

Partial-bug: #1348818

Note: There are several other unrelated unit tests that also break with
a randomized PYTHONHASHSEED, but they are not addressed here. They will
be addressed in separate patches.

Change-Id: Ia7fc2c3e605d92d8497d44e28054bdda613cebf2