category overrides are not always applied in the right order

[Sylvain Pineau]

Documentation states:

Overrides are applied in order and the last applied override is the effective override in a given test plan.

But I've found that the Bluez after-suspend jobs are not always getting the right category override.
I'm using the following field in one of our snappy test plan:

category_overrides:
    apply "2013.com.canonical.plainbox::suspend" to "2013.com.canonical.certification::after-suspend-.*"
    apply "2013.com.canonical.plainbox::suspend" to "after-suspend-.*"
    apply "2013.com.canonical.certification::after-suspend-bluetooth_bluez5_selftests" to "2013.com.canonical.certification::after-suspend-bluetooth/bluez-internal.*"

When running checkbox-cli, sometimes I see the 250 bluez tests after suspend properly grouped into their own category sometimes part of the Suspend one (the plainbox one).

Looking at plainbox code [1] in details, we are creating an override map based on collections.defaultdict(list) and looping over items(). This is not a deterministic order.
A solution used OrderedDict is needed.

[1] https://git.launchpad.net/plainbox/tree/plainbox/impl/session/state.py#n586