Merge lp://staging/~danilo/maas/django-upgrade-licensekeyform-immutable-querydict into lp://staging/~maas-committers/maas/trunk

Move osystem/distro_series combining logic to API handlers instead of LicenseKeyForm.full_clean() which now always requires such a form for distro_series field.

(follow-up from https://code.launchpad.net/~danilo/maas/django-upgrade-immutable-querydict/+merge/325995)

LicenseKeyForm fields are set up dynamically, and distro_series field is set up so that the "keys" are in the form os/series. With Django 1.11, POST data has become immutable in full_clean(), so to avoid the hack of setting internal _mutable to True, this fixes the logic so the combining of osystem and distro_series happens in the API layer instead.

And contrary to what the comment in full_clean() says, it seems API never really supported specifying only distro_series in the os/series format (a test I added for that actually failed with the old code): now it does (see test instructions below).

Testing instructions:

1. make syncdb && make sampledata && make bin/maas
2. make run
3. bin/maas login dev-1.8 http://...:5240/MAAS (use admin API key)
4. Upload a Windows image using a release that requires a license key:

  $ bin/maas dev-1.8 boot-resources create name=windows/win2012 architecture=amd64/generic filetype=ddtgz content@=/etc/passwd
5a. Browse to MAAS settings page, "License keys" section should now show up and you can still add keys ("valid" Windows key is AAAAA-BBBBB-CCCCC-DDDDD-EEEEE).

    Note that when license key validation fails, there's nothing shown in the form: I don't believe that's due to my changes, though.

5b. To test via the API, you can do:

$ bin/maas dev-1.8 boot-resources create name=windows/win2016 architecture=amd64/generic filetype=ddtgz content@=/etc/group

# Use only distro_series.
$ bin/maas dev-1.8 license-keys create distro_series=windows/win2016 license_key="AAAAA-BBBBB-11111-22222-33333"Success.
Machine-readable output follows:
{
    "resource_uri": "/MAAS/api/2.0/license-key/windows/win2016",
    "distro_series": "win2016",
    "osystem": "windows",
    "license_key": "AAAAA-BBBBB-11111-22222-33333"
}
$ bin/maas dev-1.8 license-key delete windows win2016

# Use split up osystem and distro_series.
$ bin/maas dev-1.8 license-keys create osystem=windows distro_series=win2016 license_key="AAAAA-BBBBB-11111-22222-33333"
Success.
Machine-readable output follows:
{
    "resource_uri": "/MAAS/api/2.0/license-key/windows/win2016",
    "distro_series": "win2016",
    "osystem": "windows",
    "license_key": "AAAAA-BBBBB-11111-22222-33333"
}