[SPARK-14504][SQL] Enable Oracle docker tests

## What changes were proposed in this pull request?

Enable Oracle docker tests

## How was this patch tested?

Existing tests

Author: Luciano Resende <lresende@apache.org>

Closes #12270 from lresende/oracle.

external/docker-integration-tests/pom.xml

@@ -36,8 +36,12 @@
 
   <repositories>
     <repository>
-      <id>db2</id>
+      <id>db</id>
       <url>https://app.camunda.com/nexus/content/repositories/public/</url>
+      <releases>
+        <enabled>true</enabled>
+        <checksumPolicy>warn</checksumPolicy>
+      </releases>
     </repository>
   </repositories>
 
@@ -143,14 +147,13 @@
      to use a an ojdbc jar for the testcase. The maven dependency here is commented
      because currently the maven repository does not contain the ojdbc jar mentioned.
      Once the jar is available in maven, this could be uncommented. -->
-    <!--
-      <dependency>
-        <groupId>com.oracle</groupId>
-        <artifactId>ojdbc6</artifactId>
-        <version>11.2.0.2.0</version>
-        <scope>test</scope>
-      </dependency>
-    -->
+    <dependency>
+      <groupId>com.oracle</groupId>
+      <artifactId>ojdbc6</artifactId>
+      <version>11.2.0.1.0</version>
+      <scope>test</scope>
+    </dependency>
+
     <!-- Jersey dependencies, used to override version.
      See https://github.com/apache/spark/pull/9503#issuecomment-154369560 for
      background on why we need to use a newer Jersey only in this test module;

external/docker-integration-tests/src/test/scala/org/apache/spark/sql/jdbc/OracleIntegrationSuite.scala

@@ -62,7 +62,7 @@ class OracleIntegrationSuite extends DockerJDBCIntegrationSuite with SharedSQLCo
   override def dataPreparation(conn: Connection): Unit = {
   }
 
-  ignore("SPARK-12941: String datatypes to be mapped to Varchar in Oracle") {
+  test("SPARK-12941: String datatypes to be mapped to Varchar in Oracle") {
     // create a sample dataframe with string type
     val df1 = sparkContext.parallelize(Seq(("foo"))).toDF("x")
     // write the dataframe to the oracle table tbl

project/SparkBuild.scala

@@ -367,9 +367,9 @@ object DockerIntegrationTests {
   // This serves to override the override specified in DependencyOverrides:
   lazy val settings = Seq(
     dependencyOverrides += "com.google.guava" % "guava" % "18.0",
-    resolvers ++= Seq("DB2" at "https://app.camunda.com/nexus/content/repositories/public/")
+    resolvers ++= Seq("DB2" at "https://app.camunda.com/nexus/content/repositories/public/"),
+    libraryDependencies += "com.oracle" % "ojdbc6" % "11.2.0.1.0" from "https://app.camunda.com/nexus/content/repositories/public/com/oracle/ojdbc6/11.2.0.1.0/ojdbc6-11.2.0.1.0.jar" // scalastyle:ignore
   )
-
 }
 
 /**