Fix platform selection bug.

[wxtim]

OrderedDictWithDefaults.__getitem__ will get the default, if it exists.
OrderedDictWithDefaults.get does not get the default and was causing
the platform selection to fail quietly and select localhost.

This change closes #4979

Requirements check-list

• I have read CONTRIBUTING.md and added my name as a Code Contributor.
• Contains logically grouped changes (else tidy your branch by rebase).
• Does not contain off-topic changes (use other PRs for other changes).
• Applied any dependency changes to both setup.cfg and conda-environment.yml.
• Appropriate tests are included (unit and/or functional).
• No change log entry required (why? e.g. invisible to users).
• No documentation update required.

[datamel]

I have checked this out and verified that it performs as expected. Checked tests also pass locally. Thanks @wxtim.

[MetRonnie]

Why was OrdererdDictWithDefaults.get() not fixed?

[MetRonnie]

Answer: because we don't want to mess up anything that already relies on the current behaviour.

Note that .get() still works after you've got past the "defaults_ barrier", i.e.

>>> a = OrderedDictWithDefaults()
>>> a.defaults_ =  OrderedDictWithDefaults([('job', OrderedDictWithDefaults([('batch sys', 'pbs')]))])
>>> a.get('job', {})
{}

doesn't work, but this does:

>>> a['job'].get('batch sys', None)
'pbs'

because at that point you're accessing the primary dict of an OrderedDictWithDefaults as opposed to its defaults