Overnigth jobs are failing: 'Node' object has no attribute 'get'

As in #984 (closed), this seems a problem with the master using a plugin from bst-external, which is meant to be bst-1.2 only at the moment.

A fix might be done in bst-external keeping backward compatibility

Job #190989239 failed for 19bbc4c4:

      File "/builds/BuildStream/buildstream/src/bst-ext/bst_external/elements/flatpak_image.py", line 51, in configure
        for section, pairs in node.get('metadata').items():
    AttributeError: 'Node' object has no attribute 'get'