Add support for Pivotal HD in the Maven build: SPARK-1992

Allow Spark to build against particular Pivotal HD distributions. For example to build Spark against Pivotal HD 2.0.1 one can run:
```
mvn -Pyarn -Phadoop-2.2 -Dhadoop.version=2.2.0-gphd-3.0.1.0 -DskipTests clean package
```

Author: tzolov <christian.tzolov@gmail.com>

Closes #942 from tzolov/master and squashes the following commits:

bc3e05a [tzolov] Add support for Pivotal HD in the Maven build and SBT build: [SPARK-1992]

pom.xml

@@ -206,6 +206,17 @@
         <enabled>false</enabled>
       </snapshots>
     </repository>
+    <repository>
+      <id>spring-releases</id>
+      <name>Spring Release Repository</name>
+      <url>http://repo.spring.io/libs-release</url> 
+      <releases>
+        <enabled>true</enabled>
+      </releases>
+      <snapshots>
+        <enabled>false</enabled>
+      </snapshots>
+    </repository> 
   </repositories>
 
   <dependencyManagement>

project/SparkBuild.scala

@@ -212,6 +212,7 @@ object SparkBuild extends Build {
       "JBoss Repository"     at "https://repository.jboss.org/nexus/content/repositories/releases/",
       "MQTT Repository"      at "https://repo.eclipse.org/content/repositories/paho-releases/",
       "Cloudera Repository"  at "http://repository.cloudera.com/artifactory/cloudera-repos/",
+      "Pivotal Repository"   at "http://repo.spring.io/libs-release/",
       // For Sonatype publishing
       // "sonatype-snapshots"   at "https://oss.sonatype.org/content/repositories/snapshots",
       // "sonatype-staging"     at "https://oss.sonatype.org/service/local/staging/deploy/maven2/",