previous mkDefault did not work as expected,
as it did not overwrite the original submodule's defaults when the user
did not specify any custom options at all.
if the nginx option is used.
Noted that either webServerUser or nginx option is mandatory.
Also introduce an assertion if both are not set,
and a warning if both are set.
Resolves#27704.
instead of redeclaring part of the options. Backward-compatible change.
This gives the same flexibility to the user as nginx itself.
This also resolves the piwik module break from nginx' enableSSL introduction from #27426.
