[CI] plugins:repository-s3 build fails for third-party-tests

[vladimirdolzhenko]

seems like a result of 32d67ef commit
https://elasticsearch-ci.elastic.co/job/elastic+elasticsearch+master+third-party-tests/9/console

08:43:33 > Configure project :plugins:repository-s3
08:43:33 Evaluating project ':plugins:repository-s3' using build file '/var/lib/jenkins/workspace/elastic+elasticsearch+master+third-party-tests/plugins/repository-s3/build.gradle'.
08:43:33 
08:43:33 * What went wrong:
08:43:33 A problem occurred evaluating project ':plugins:repository-s3'.
08:43:33 > not all options specified to run against external S3 service

[elasticmachine]

Pinging @elastic/es-distributed

[vladimirdolzhenko]

related build failure : https://elasticsearch-ci.elastic.co/job/elastic+elasticsearch+6.x+third-party-tests/9/console

* What went wrong:
Execution failed for task ':plugins:repository-s3:qa:amazon-s3:integTestCluster#addToKeystore#s3.client.integration_test_temporary.access_key'.
> Cannot invoke method getBytes() on null object

none of amazon_s3_access_key, amazon_s3_secret_key, amazon_s3_access_key_temporary, amazon_s3_secret_key_temporary or amazon_s3_session_token_temporary are configured for the build:
https://elasticsearch-ci.elastic.co/job/elastic+elasticsearch+6.x+third-party-tests/9/injectedEnvVars/

[bleskes]

Another example, if need be: https://elasticsearch-ci.elastic.co/job/elastic+elasticsearch+master+third-party-tests/16/console