Parse Monasca Log API timestamps correctly

By parsing the creation_time timestamp in Logstash, Elasticsearch
can parse it correctly. This closes a bug where the creation_time
timestamp was shown as a date shortly after the epoch (1970) when
viewed in Kibana.

Closes-Bug: #1816585

Change-Id: I00decfe94607845ef0eae9bec631a0e729aac3fa