[ML] Cannot create datafeed that uses aggs

[sophiec20]

Elasticsearch version (bin/elasticsearch --version):
Version: 6.6.0-SNAPSHOT, Build: default/zip/928396e/2018-11-30T11:04:13.733240Z, JVM: 9.0.4

Note: also found on master, neither are a released version

Plugins installed: n/a

JVM version (java -version):

java version "9.0.4"
Java(TM) SE Runtime Environment (build 9.0.4+11)
Java HotSpot(TM) 64-Bit Server VM (build 9.0.4+11, mixed mode)

OS version (uname -a if on a Unix-like system):
Mac 17.7.0 Darwin Kernel Version 17.7.0

Description of the problem including expected versus actual behavior:
Datafeed creation fails for jobs that contain aggregations. Found for all agg jobs within in qa framework. Occurs since
#36117

[2018-12-04T19:50:29.948] [FAILED] [metricbeat-derivatives] "Derivative of Metricbeat bytes in" in method scenarioJobMetricbeat [7.0.0-3676]

Reason: An error occured during datafeed create (id 'datafeed-metricbeat-derivatives_20181204-1949_700_3676')
Error response: {"reason":"Unexpected token START_OBJECT in [aggs]: aggregations definitions must start with the name of the aggregation.","col":1,"line":1,"type":"parsing_exception","root_cause":[{"reason":"Unexpected token START_OBJECT in [aggs]: aggregations definitions must start with the name of the aggregation.","col":1,"line":1,"type":"parsing_exception"}]}

Steps to reproduce:
Take above job config from qa framework.

[elasticmachine]

Pinging @elastic/ml-core