[Datumaro] Mandatory sorting for black color in voc masks

[zhiltsov-max]

Motivation and context

Fixes #2032

• Removed guess label map type, which combined voc and source labels. Default is set to source.
• Black color is automatically added. It is automatically sorted to be the first in the list. Default name is background.

How has this been tested?

Unit tests

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 7118

• 20 of 22 (90.91%) changed or added relevant lines in 2 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage increased (+0.01%) to 69.747%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
datumaro/datumaro/plugins/voc_format/converter.py	16	17	94.12%
datumaro/datumaro/plugins/voc_format/format.py	4	5	80.0%

Files with Coverage Reduction	New Missed Lines	%
datumaro/datumaro/plugins/voc_format/converter.py	1	89.9%

Totals
Change from base Build 7115:	0.01%
Covered Lines:	11754
Relevant Lines:	16396

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💛 - Coveralls

[nmanovic]

LGTM