Fix HDFS access bug with assembly build.

Due to this change in HDFS:
https://issues.apache.org/jira/browse/HADOOP-7549

there is a bug when using the new assembly builds. The symptom is that any HDFS access
results in an exception saying "No filesystem for scheme 'hdfs'". This adds a merge
strategy in the assembly build which fixes the problem.

assembly/pom.xml

@@ -103,6 +103,12 @@
               <goal>shade</goal>
             </goals>
             <configuration>
+              <transformers>
+                <transformer implementation="org.apache.maven.plugins.shade.resource.ServicesResourceTransformer"/>
+                <transformer implementation="org.apache.maven.plugins.shade.resource.AppendingTransformer">
+                  <resource>META-INF/services/org.apache.hadoop.fs.FileSystem</resource>
+                </transformer>
+              </transformers>
               <transformers>
                 <transformer implementation="org.apache.maven.plugins.shade.resource.ServicesResourceTransformer"/>
                 <transformer implementation="org.apache.maven.plugins.shade.resource.AppendingTransformer">

project/SparkBuild.scala

@@ -303,6 +303,7 @@ object SparkBuild extends Build {
     mergeStrategy in assembly := {
       case m if m.toLowerCase.endsWith("manifest.mf") => MergeStrategy.discard
       case m if m.toLowerCase.matches("meta-inf.*\\.sf$") => MergeStrategy.discard
+      case "META-INF/services/org.apache.hadoop.fs.FileSystem" => MergeStrategy.concat
       case "reference.conf" => MergeStrategy.concat
       case _ => MergeStrategy.first
     }