config: keep dicts in config ordered

YAML mappings are converted in Python as dict. The order of the keys in dicts returned by load_from_envvar() were not stable from one Python run to the next.

Since Python 3.6, dicts are ordered by default, and yaml.safe_load() will always return keys in the same order. The culprit was merge_configs which created a set (which are unordered) of keys to perform the merge.