Feature to add a postgres service for labs

[netoisc]

• add postgres dependency for notebooks
• add new service definition in docker-compose
• add new ansible flag to indicate when to use this service

How to use with python:

import psycopg2
conn = psycopg2.connect(host="postgres-labs", database="labs", user="postgres", password="postgres")

[codecov]

Codecov Report

Merging #75 into master will not change coverage.
The diff coverage is n/a.

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##           master      #75   +/-   ##
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  Coverage   58.16%   58.16%           
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  Files          11       11           
  Lines         710      710           
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  Hits          413      413           
  Misses        297      297           

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[jgwerner]

We need to also add a section to the Readme so the user is aware of this configuration option. We would add instructions to the user guide docs separately, so the README.md should have config options to enable or disable the service along with some basic instructions on how to access the DB from the notebook(s).

[jgwerner]

LGTM