FELIX-4330 - Make SSL headers configurable:

- applied a slightly different approach as suggested by [~fmeschbe] by splitting the header
  name and value into 2 different configuration properties allowing any header variant to
  be supported;
- also made the header used to obtain forwarded client certificates configurable, as it 
  appears to have at least one other representation in the wild;
- added metatype description for the managed service to document the configuration properties.



git-svn-id: https://svn.apache.org/repos/asf/felix/trunk@1596230 13f79535-47bb-0310-9956-ffa450edef68