Stackdriver supports structured JSON payloads in addition to simple
plain-text payloads.
This commit introduces a new feature in which journaldriver will
attempt to parse incoming log messages into JSON vaues and forward
them as structured payloads if they are JSON objects.
Messages that can not be parsed into JSON objects will continue to be
forwarded as plain text messages.
This implements the "meat" of the initial version of journaldriver.
Records from journald are converted into the representation required
by Stackdriver and forwarded to the API.
In this initial version journaldriver is only supported on instances
running on GCP.
