Fix mapping of boolean record_mode to underlying vcr modes

[v-iam]

This PR fixes #21 by passing an appropriate string for VCR()'s record_mode parameter rather than a boolean like it was doing before.

[msftclas]

@v-iam,
Thanks for your contribution as a Microsoft full-time employee or intern. You do not need to sign a CLA.
Thanks,
Microsoft Pull Request Bot

[codecov-io]

Codecov Report

Merging #22 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #22   +/-   ##
=======================================
  Coverage   48.16%   48.16%           
=======================================
  Files          10       10           
  Lines         382      382           
  Branches       58       58           
=======================================
  Hits          184      184           
  Misses        194      194           
  Partials        4        4

Impacted Files	Coverage Δ
src/azure_devtools/scenario_tests/base.py	38.33% <ø> (ø)	⬆️

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[lmazuel]

Duplicate of merged #30