[SPARK-8397] [SQL] Allow custom configuration for TestHive

We encourage people to use TestHive in unit tests, because it's
impossible to create more than one HiveContext within one process. The
current implementation locks people into using a local[2] SparkContext
underlying their HiveContext.  We should make it possible to override
this using a system property so that people can test against
local-cluster or remote spark clusters to make their tests more
realistic.

Author: Punya Biswal <pbiswal@palantir.com>

Closes #6844 from punya/feature/SPARK-8397 and squashes the following commits:

97ef394 [Punya Biswal] [SPARK-8397][SQL] Allow custom configuration for TestHive

sql/hive/src/main/scala/org/apache/spark/sql/hive/test/TestHive.scala

@@ -49,7 +49,7 @@ import scala.collection.JavaConversions._
 object TestHive
   extends TestHiveContext(
     new SparkContext(
-      "local[2]",
+      System.getProperty("spark.sql.test.master", "local[2]"),
       "TestSQLContext",
       new SparkConf()
         .set("spark.sql.test", "")