[SPARK-5574] use given name prefix in dir

https://issues.apache.org/jira/browse/SPARK-5574

very minor, doesn't effect external behavior at all.
Note that after this change, some of these dirs no longer will have "spark" in the name at all.  I could change those locations that do pass in a name prefix to also include "spark", eg. "blockmgr" -> "spark-blockmgr"

Author: Imran Rashid <irashid@cloudera.com>

Closes #4344 from squito/SPARK-5574 and squashes the following commits:

33a84fe [Imran Rashid] use given name prefix in dir

core/src/main/scala/org/apache/spark/util/Utils.scala

@@ -279,7 +279,7 @@ private[spark] object Utils extends Logging {
           maxAttempts + " attempts!")
       }
       try {
-        dir = new File(root, "spark-" + UUID.randomUUID.toString)
+        dir = new File(root, namePrefix + "-" + UUID.randomUUID.toString)
         if (dir.exists() || !dir.mkdirs()) {
           dir = null
         } else {