add exempt-namespace-prefix to helm chart

[wolffberg]

What this PR does / why we need it:
This PR is going to add support for setting exempt-namespace-prefix arguments to the deployment of gatekeeper-controller-manager from the values YAML file.

Special notes for your reviewer:

• This PR is based on changes in set exempt-namespaces to gatekeeper-controller-manager from values YAML #1408
• Small spacing fixes have been made to the README.md tables

[ritazh]

@wolffberg Thank you for the PR! Can you please fix the failing DCO check? (instructions are in the Details link)

[wolffberg]

Ah sorry, missed that one. All done 🎉

[codecov-commenter]

Codecov Report

Merging #1523 (b4e453e) into master (c91112c) will increase coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1523      +/-   ##
==========================================
+ Coverage   51.86%   51.90%   +0.03%     
==========================================
  Files          86       86              
  Lines        7812     7812              
==========================================
+ Hits         4052     4055       +3     
+ Misses       3404     3402       -2     
+ Partials      356      355       -1     

Flag	Coverage Δ
unittests	51.90% <ø> (+0.03%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
...onstrainttemplate/constrainttemplate_controller.go	55.50% <0.00%> (ø)
pkg/controller/mutators/core/controller_core.go	55.44% <0.00%> (+1.55%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update c91112c...b4e453e. Read the comment docs.

[maxsmythe]

LGTM

[ritazh]

Can you pls rerun make manifests then checkin all the changes in manifest_staging? thank you!

[wolffberg]

Done