Clarify TLS configuration in HTTP

**Issue by [jrudolph](https://github.com/jrudolph)**
_Saturday Dec 26, 2015 at 09:50 GMT_
_Originally opened as https://github.com/akka/akka/issues/19294_

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i.e. it should be clear what the relation between HttpsContext and sslConfig should be. HttpsContext allowed configuration of TLS settings for every single one of the `Http` APIs. It seems that now the global SSL config may override some settings.

IMO, in HTTP, there should only be one mechanism to configure TLS and this configuration shouldn't be touched by some global settings.

See also https://gitter.im/akka/akka?at=567c0e150199d70069df9867, #19264, and #19275.

Start by reading the HttpsContext and sslConfig configuration paths used by the Http APIs, then compare the behavior discussed in #19264, #19275, and the linked Gitter conversation. Determine which TLS configuration mechanism should be authoritative and define how global settings and per-API settings should interact; the issue is done when that relationship is unambiguous and consistently implemented.

Written by the indexing model from the issue text.