[Datumaro] Fix convert command

[zhiltsov-max]

Motivation and context

Updates datum convert command after converter interface changes.

How has this been tested?

Manual test

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 6604

• 0 of 3 (0.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.03%) to 64.973%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
datumaro/datumaro/cli/commands/convert.py	0	3	0.0%

Totals
Change from base Build 6575:	0.03%
Covered Lines:	11087
Relevant Lines:	16658

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💛 - Coveralls