Added hdfs_skip option.

This makes it extra clear that, when launched via hadoop and running
in a mapper task, H2O does not load any HDFS library itself.
Instead, H2O inherits the HDFS library from the mapper task.

hadoop/src/main/java/water/hadoop/h2omapper.java

@@ -344,8 +344,12 @@ private int run2(Context context) throws IOException, InterruptedException {
     String localPortString = Integer.toString(ss.getLocalPort());
 
     String[] args = {
+            // Options used by H2O.
             "-ice_root", ice_root,
             "-name", jobtrackerName,
+            "-hdfs_skip",
+
+            // Options passed through to UserMain for configuring the EmbeddedH2OConfig.
             "-driverip", driverIp,
             "-driverport", driverPortString,
             "-mapperport", localPortString

src/main/java/water/H2O.java

@@ -596,6 +596,7 @@ public static class OptArgs extends Arguments.Opt {
     public String hdfs; // HDFS backend
     public String hdfs_version; // version of the filesystem
     public String hdfs_config; // configuration file of the HDFS
+    public String hdfs_skip = null; // used by hadoop driver to not unpack and load any hdfs jar file at runtime.
     public String aws_credentials; // properties file for aws credentials
     public String keepice; // Do not delete ice on startup
     public String soft = null; // soft launch for demos

src/main/java/water/persist/HdfsLoader.java

@@ -14,6 +14,15 @@ public class HdfsLoader {
   private static final String MAPRFS_HDFS_VERSION = "mapr2.1.3";
 
   public static void loadJars() {
+    if (H2O.OPT_ARGS.hdfs_skip != null) {
+      // When H2O is launched by hadoop itself, it should use the HDFS library that
+      // the hadoop mapper task picks up by default.
+      //
+      // Do not load any hadoop jar that is packed with H2O.
+      Log.info("H2O was started by Hadoop; inheriting HDFS library from mapper task.");
+      return;
+    }
+
     // Load the HDFS backend for existing hadoop installations.
     // FIX! hadoop/mapr supports other variants? also why isn't port an option on mapr, and why volume?
     // port should be optional