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support arbitrary objects for the input data.

Domain.set_param and Domain.get_param convenience methods

It is of more utility in symbiosis-common, and we could all it elsewhere
when templating.

No functional changes made, just minor updates to the comments.

Also fix up the DKIM test to reset the hostname if it thinks it is
localhost, as this is explicitly disallowed in the code (for some
reason).

This reverts commit dc3692ea.