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.
diff --git a/test/cpp/interop/server.cc b/test/cpp/interop/server.cc
index 561b134..5b5c354 100644
--- a/test/cpp/interop/server.cc
+++ b/test/cpp/interop/server.cc
@@ -203,11 +203,7 @@
   builder.RegisterService(service.service());
   if (FLAGS_enable_ssl) {
     SslServerCredentialsOptions ssl_opts = {
-        "",
-        {reinterpret_cast<const char*>(test_server1_key),
-         test_server1_key_size},
-        {reinterpret_cast<const char*>(test_server1_cert),
-         test_server1_cert_size}};
+        "", {{test_server1_key, test_server1_cert}}};
     std::shared_ptr<ServerCredentials> creds =
         ServerCredentialsFactory::SslCredentials(ssl_opts);
     builder.SetCredentials(creds);