Update Python C wrapping layers to new credentials API

Core creds api change

- switches credentials to channel credentials
- removes usages of other credentials classes that would are not
  currently used in the ruby surface

Node wrapping of core credentials API change

Update C# to the new C core auth API

PHP: Core credentials API refactoring

s/grpc_credentials/grpc_channel_credentials for ObjC

The composite channel credentials creation does not change the passed-in
channel creds anymore.

Add empty_stream Python interop test.

Accept --use_tls=true and --use_test_ca=true in python interop tests.

cleanup dockerfiles and scripts for old interop infrastructure

Conflicts:
	.gitignore
	gRPC.podspec

Fix release conflict for release-0_11

Make Node coverage reporting only report on actual Node source files

Restrict number of iterations for UnaryCallPerformance test

Make ruby RpcServer stop waiting for calls when it gets null calls

Revert global_env usage for build_interop_image job