[CI] waitForPendingTasks caused all DatafeedJobsRestIT tests to fail

[droberts195]

In https://elasticsearch-ci.elastic.co/job/elastic+elasticsearch+master+intake/585/console every test in the DatafeedJobsRestIT suite failed with the following exception:

> Throwable #1: java.lang.AssertionError: 1 active tasks found:
   > cluster:monitor/discovered_nodes uGtrOM6aQcqAFKqufcXp4Q:1     -                            direct    1544176084851 09:48:04 9m          127.0.0.1 node-0 

This is a side effect of zen2 discovery. Certain ML integration suites wait for pending tasks to complete in between tests. This is so that side effects of one ML test do not interfere with the next test. The long-running zen2 task needs to be excluded from the check.

[elasticmachine]

Pinging @elastic/ml-core