Do not assume order of get_sync_data_metering response elements

Do not assume order of get_sync_data_metering response elements

This fixes the test_add_metering_label_rpc_call[1] unit tests that breaks
with a randomized PYTHONHASHSEED (see the bug report).

The test assumed that the get_sync_data_metering[2] had response
elements in a particular order. Found with PYTHONHASHSEED=1.

The fix refactors the test_add_metering_label_rpc_call test case to handle
an unsorted get_sync_data_metering response. The fix defines the class
UnorderedList[3] which is equal to any permutation of itself.

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.

[1] neutron.tests.unit.services.metering.test_metering_plugin:
 TestMeteringPluginL3AgentScheduler
 TestMeteringPluginL3AgentSchedulerServicePlugin
[2] neutron.services.metering.metering_plugin.MeteringPlugin
[3] neutron.tests.tools

Change-Id: I5d42b827bc72dcacd38eaa2377ce16c47a9e7dbb