fetch STS endpoint from properties

Even when providing `withRegion`, the `AWSSecurityTokenServiceClientBuilder` points the client to us-east-1. This fails in cases where you are using a VPC endpoint and access to the Internet is limited.

This change looks up the property `aws.sts.endpoint` and uses that or `https://sts.amazonaws.com` to pass to `.withEndpointConfiguration`

Closes #58

Author: jhulten

src/main/scala/com/audienceproject/spark/dynamodb/connector/DynamoConnector.scala

@@ -75,10 +75,12 @@ private[dynamodb] trait DynamoConnector {
       **/
     private def getCredentials(chosenRegion: String, roleArn: Option[String]) = {
         roleArn.map(arn => {
+            val stsEndpoint = Option(System.getProperty("aws.sts.endpoint")).getOrElse("https://sts.amazonaws.com")
             val stsClient = AWSSecurityTokenServiceClientBuilder
                 .standard()
                 .withCredentials(new DefaultAWSCredentialsProviderChain)
                 .withRegion(chosenRegion)
+                .withEndpointConfiguration(new EndpointConfiguration(stsEndpoint, chosenRegion))
                 .build()
             val assumeRoleResult = stsClient.assumeRole(
                 new AssumeRoleRequest()