Revision 1596230

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.