Changing the SSL (Server) Credentials API.

- Changed the unsigned char * + size to NULL terminated strings which
  makes sense for the PEM format. I may change TSI later (but the impact
  will hopefully be much more limited).
- Added a way to pass multiple key/cert pairs to servers which is needed
  when hosting more than one domain.
- Removed the C++ SSL credentials tests as we are going to have an
  option to not specify the roots which will then be derived from the
  environment (well-known platform dependent locations and/or
  environment variable).
- Fixed the php build which is the only one added in the run_test.py.
  This change will certainly break node, python and ruby.