[SPARK-2746] Set SBT_MAVEN_PROFILES only when it is not set explicitly by the user.

Author: Reynold Xin <rxin@apache.org>

Closes #1655 from rxin/SBT_MAVEN_PROFILES and squashes the following commits:

b268c4b [Reynold Xin] [SPARK-2746] Set SBT_MAVEN_PROFILES only when it is not set explicitly by the user.

dev/run-tests

@@ -21,7 +21,10 @@
 FWDIR="$(cd `dirname $0`/..; pwd)"
 cd $FWDIR
 
-export SBT_MAVEN_PROFILES="-Pyarn -Phadoop-2.3 -Dhadoop.version=2.3.0"
+if [ -z "$SBT_MAVEN_PROFILES" ]; then
+  export SBT_MAVEN_PROFILES="-Pyarn -Phadoop-2.3 -Dhadoop.version=2.3.0"
+fi
+echo "SBT_MAVEN_PROFILES=\"$SBT_MAVEN_PROFILES\""
 
 # Remove work directory
 rm -rf ./work