Tests: check maven central download service is up before running plugin manager test

Though we check that internet connection is fine and that the service is available, it could happen that some services are not available.

```java
assumeTrue(isDownloadServiceWorking("search.maven.org", 80, "/"));
singlePluginInstallAndRemove("org.elasticsearch/elasticsearch-transport-thrift/2.4.0", null);
```

In that case, the first check was successful but downloading thrift plugin from maven central download services was not successful.

This is not an issue with the plugin manager itself but sometimes the test could fail.

Adding a check that actual download service in Maven central works:

```java
assumeTrue(isDownloadServiceWorking("repo1.maven.org", 443, "/maven2/org/elasticsearch/elasticsearch-transport-thrift/2.4.0/elasticsearch-transport-thrift-2.4.0.pom"));
```

(cherry picked from commit 6ef6109)