Fixed subscribe topic bug

[TheBurchLog]

When using the topic variable, it would appear 3 times in the topic list.

[codecov]

Codecov Report

Attention: 81 lines in your changes are missing coverage. Please review.

Comparison is base (a738e1c) 94.26% compared to head (1c1e177) 92.39%.
Report is 10 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #417      +/-   ##
===========================================
- Coverage    94.26%   92.39%   -1.87%     
===========================================
  Files           26       28       +2     
  Lines         3452     3644     +192     
===========================================
+ Hits          3254     3367     +113     
- Misses         198      277      +79     

Files	Coverage Δ
brewtils/__init__.py	81.25% <100.00%> (+2.67%)	⬆️
brewtils/__version__.py	100.00% <100.00%> (ø)
brewtils/models.py	90.36% <100.00%> (+0.04%)	⬆️
brewtils/rest/client.py	93.86% <100.00%> (+0.02%)	⬆️
brewtils/schemas.py	99.17% <100.00%> (+<0.01%)	⬆️
brewtils/plugin.py	96.27% <50.00%> (-0.27%)	⬇️
brewtils/rest/system_client.py	91.20% <44.44%> (-8.15%)	⬇️
brewtils/auto_decorator.py	23.80% <23.80%> (ø)
brewtils/rest/publish_client.py	32.00% <32.00%> (ø)
brewtils/decorators.py	86.92% <71.68%> (-9.02%)	⬇️

... and 1 file with indirect coverage changes

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