interop-testing: add alpnagent to support okhttp in test client

[ericgribkoff]

In response to grpc/grpc#11258 / #3032, we should run our full suite of interop tests using OkHttp. As best I can tell, OkHttp cannot use ALPN unless it has the ALPN agent, which was removed a while ago from the build rule, back when we were still using the boot classpath. This adds it back in, so that the --use_okhttp=true option in the test client is able to use TLS.

[ejona86]

This does not work if you move the install files to a different machine or clear the gradle cache. It has a hard-coded path:

$ grep javaagent interop-testing/build/install/grpc-interop-testing/bin/test-client
DEFAULT_JVM_OPTS='"-javaagent:/usr/local/google/home/ejona/.gradle/caches/modules-2/files-2.1/org.mortbay.jetty.alpn/jetty-alpn-agent/2.0.6/ecca008cfd140e088590c0f2e0f223523cdb18d2/jetty-alpn-agent-2.0.6.jar"'

[ericgribkoff]

AFAIK, the only alternative to using Gradle's defaultJvmOpts with the hard-coded path in the generated shell script is to require the user to specify the path to Jetty ALPN on the command-line, e.g., env JAVA_OPTS=-javaagent:pathTo/jetty-alpn-agent-2.0.6.jar build/install/grpc-interop-testing/bin/test-client --server_host_override=foo.test.google.fr --use_test_ca=true --use_okhttp=true. Is this alternative the preferred approach? If so, I should at least add some message to the test client output explaining that this is required. Right now the combination of tls and okhttp never works.

[ericgribkoff]

@ejona86 I updated build.gradle to avoid having hard-coded paths in the startup scripts. This requires a little bit of hackery to get the $APP_PATH and %APP_PATH% variables working correctly on *nix and Windows startup scripts.