[CELEBORN-775][FOLLOWUP] Fix executorCores calculation in SparkShuffl…

…eManager for Spark2 local mode

### What changes were proposed in this pull request?

### Why are the changes needed?
CELEBORN-775 The `executorCores` method was introduced in Spark2, but it was not used.

### Does this PR introduce _any_ user-facing change?
No

### How was this patch tested?
GA

Closes apache#2562 from cxzl25/CELEBORN-775-FOLLOWUP.

Authored-by: sychen <sychen@ctrip.com>
Signed-off-by: SteNicholas <programgeek@163.com>

client-spark/spark-2/src/main/java/org/apache/spark/shuffle/celeborn/SparkShuffleManager.java

@@ -69,7 +69,7 @@ public SparkShuffleManager(SparkConf conf, boolean isDriver) {
     this.conf = conf;
     this.isDriver = isDriver;
     this.celebornConf = SparkUtils.fromSparkConf(conf);
-    this.cores = conf.getInt(SparkLauncher.EXECUTOR_CORES, 1);
+    this.cores = executorCores(conf);
     this.fallbackPolicyRunner = new CelebornShuffleFallbackPolicyRunner(celebornConf);
     this.sendBufferPoolCheckInterval = celebornConf.clientPushSendBufferPoolExpireCheckInterval();
     this.sendBufferPoolExpireTimeout = celebornConf.clientPushSendBufferPoolExpireTimeout();