Fixed post-reviews script to use 'rbt post' when 'post-review' is not
available.

Review: https://reviews.apache.org/r/20284
[MESOS-1259]:Enrich the Java Docs in the src/java files. -- Executor.java
[MESOS-1259]:Enrich the Java Docs in the src/java files. -- ExecutorDriver.java
[MESOS-1259]:Enrich the Java Docs in the src/java files. -- Log.java
[MESOS-1259]:Enrich the Java Docs in the src/java files. -- Log.java
[MESOS-1259]:Enrich the Java Docs in the src/java files. -- MesosSchedulerDriver.java
[MESOS-1259]:Enrich the Java Docs in the src/java files. -- Scheduler.java
[MESOS-1259]:Enrich the Java Docs in the src/java files. -- SchedulerDriver.java
[MESOS-1259]:Enrich the Java Docs in the src/java files. -- State.java
[MESOS-1259]:Enrich the Java Docs in the src/java files. -- Variable.java
[MESOS-1259]:Enrich the Java Docs in the src/java files. -- ZooKeeperState.java