[SPARK-18050][SQL] do not create default database if it already exists

## What changes were proposed in this pull request?

When we try to create the default database, we ask hive to do nothing if it already exists. However, Hive will log an error message instead of doing nothing, and the error message is quite annoying and confusing.

In this PR, we only create default database if it doesn't exist.

## How was this patch tested?

N/A

Author: Wenchen Fan <wenchen@databricks.com>

Closes #15993 from cloud-fan/default-db.

sql/core/src/main/scala/org/apache/spark/sql/internal/SharedState.scala

@@ -92,8 +92,12 @@ private[sql] class SharedState(val sparkContext: SparkContext) extends Logging {
   {
     val defaultDbDefinition = CatalogDatabase(
       SessionCatalog.DEFAULT_DATABASE, "default database", warehousePath, Map())
-    // Initialize default database if it doesn't already exist
-    externalCatalog.createDatabase(defaultDbDefinition, ignoreIfExists = true)
+    // Initialize default database if it doesn't exist
+    if (!externalCatalog.databaseExists(SessionCatalog.DEFAULT_DATABASE)) {
+      // There may be another Spark application creating default database at the same time, here we
+      // set `ignoreIfExists = true` to avoid `DatabaseAlreadyExists` exception.
+      externalCatalog.createDatabase(defaultDbDefinition, ignoreIfExists = true)
+    }
   }
 
   /**