Enable buffering to file for Monasca logs

This enables buffering to file, rather than memory for Monasca logs.
A dedicated docker volume is used for the file buffer. If a post
to the Monasca Log API fails, retries will be made using an exponential
backoff algorithm with a maximum retry interval of 30mins. The maximum
interval is set relatively low to try and reduce the risk of large
buffers accumulating, and therefore the risk of overloading the Monasca
Log API.

Closes-Bug: #1855700
Change-Id: Ib5286e9dbaf2bc92d2f4960b2131223ab5dbdbec