feat: adding option to use cert-manager

[gvicentin]

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[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 73.17073% with 11 lines in your changes missing coverage. Please review.

Project coverage is 72.78%. Comparing base (1e295e0) to head (1515140).
Report is 11 commits behind head on master.

Files	Patch %	Lines
tsuru/client/certificate.go	73.17%	6 Missing and 5 partials ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #229      +/-   ##
==========================================
+ Coverage   70.89%   72.78%   +1.89%     
==========================================
  Files          55       54       -1     
  Lines        8809     9689     +880     
==========================================
+ Hits         6245     7052     +807     
- Misses       1675     1737      +62     
- Partials      889      900      +11     

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