Only add SSLProxyEngine in proxy mode and when SSL is enabled for the web (fixes #6269)

J. Daniel Schmidtrequested to merge
jdsn/ispconfig3:6269-redirects-render-invalid-apache-config-on-non-ssl-web-server-since-mr-1521-jdsn into develop

SSLProxyEngine is neither needed for http(s) redirects nor for non-SSL proxy configs. Thus excluding it from these cases.

Based on a fix by Thom thom@amsterdamtech.nl

Offering this MR as alternative to !1548 (closed)

Closes #6269 (closed)

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