python nexmark benchmark suite launcher

Create the launcher as well as the configurations for benchmark entry program.

For reference:

the java launcher: https://github.com/apache/beam/blob/master/sdks/java/testing/nexmark/src/main/java/org/apache/beam/sdk/nexmark/NexmarkLauncher.java

and its configuration: https://github.com/apache/beam/blob/master/sdks/java/testing/nexmark/src/main/java/org/apache/beam/sdk/nexmark/NexmarkOptions.java