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.
