Fix de_client call

[vitodb]

This is required to allow de_client to get its output returned as text string rather then using a logger.
This fixes an issue when using decisionengine_modules/glideinwms/configure_gwms_frontend.py to generate GWMS configuration from glideinwms.libsonnet
Thanks to Kyle for the help to get the issue fixed.

[codecov]

Codecov Report

Base: 47.41% // Head: 47.41% // No change to project coverage 👍

Coverage data is based on head (a6744f1) compared to base (f2057d1).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #436   +/-   ##
=======================================
  Coverage   47.41%   47.41%           
=======================================
  Files          54       54           
  Lines        2902     2902           
  Branches      523      523           
=======================================
  Hits         1376     1376           
  Misses       1426     1426           
  Partials      100      100           

Flag	Coverage Δ
python-3.6	?
python-3.9	47.41% <100.00%> (ø)

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Impacted Files	Coverage Δ
...ecisionengine_modules/glideinwms/DEConfigSource.py	100.00% <100.00%> (ø)

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