Continuation of https://github.com/opencv/cvat/pull/1641

[412b]

Added env variables for Postgres DB and user names

Motivation and context

Configurable DB name and user name are standard

How has this been tested?

No special testing is needed, fallback in case of missing env vars is the previous value

Checklist

• I submit my changes into the develop branch
• I have added description of my changes into CHANGELOG file
• I have updated the documentation accordingly
• I have added tests to cover my changes
• I have linked related issues (read github docs)
• I have increased versions of npm packages if it is necessary (cvat-canvas,
  cvat-core, cvat-data and cvat-ui)

License

• I submit my code changes under the same MIT License that covers the project.
  Feel free to contact the maintainers if that's a concern.
• I have updated the license header for each file (see an example below)

# Copyright (C) 2020 Intel Corporation
#
# SPDX-License-Identifier: MIT

[coveralls]

Pull Request Test Coverage Report for Build 6095

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 2 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-0.009%) to 65.416%

---

Files with Coverage Reduction	New Missed Lines	%
cvat/apps/engine/media_extractors.py	2	77.06%

Totals
Change from base Build 6062:	-0.009%
Covered Lines:	10994
Relevant Lines:	16395

---

💛 - Coveralls

[nmanovic]

LGTM, thanks for the PR