Refactor RepositoryCredentialsTests (#45919)

This commit refactors the S3 credentials tests in 
RepositoryCredentialsTests so that it now uses a single 
node (ESSingleNodeTestCase) to test how secure/insecure 
credentials are overriding each other. Using a single node 
makes it much easier to understand what each test is actually 
testing and IMO better reflect how things are initialized.

It also allows to fold into this class the test 
testInsecureRepositoryCredentials which was wrongly located 
in S3BlobStoreRepositoryTests. By moving this test away, the 
S3BlobStoreRepositoryTests class does not need the 
allow_insecure_settings option anymore and thus can be 
executed as part of the usual gradle test task.

plugins/repository-s3/build.gradle

@@ -67,15 +67,13 @@ bundlePlugin {
 
 task testRepositoryCreds(type: Test) {
   include '**/RepositoryCredentialsTests.class'
-  include '**/S3BlobStoreRepositoryTests.class'
   systemProperty 'es.allow_insecure_settings', 'true'
 }
 check.dependsOn(testRepositoryCreds)
 
 test {
   // these are tested explicitly in separate test tasks
-  exclude '**/*CredentialsTests.class'
-  exclude '**/S3BlobStoreRepositoryTests.class'
+  exclude '**/RepositoryCredentialsTests.class'
   exclude '**/S3RepositoryThirdPartyTests.class'
 }