Interop stress tests

Removing coveralls badge, as we're no longer using it.

Added per-test XML reports for Node tests on Jenkins

Stop using SSL_CERT_FILE env for C# interop tests

Add test for CPU functions. 

PHP: remove unused file

Make Node interop client use default roots file path

Add an indicator for server and client languages.

Progress towards green interop tests

μChannels implementation.

Generate html report for interop tests

PHP: add more unit test for code coverage analysis

Consistent interop cmdline args

Fix python compute_engine_creds and oauth2_auth_token interop tests.