[common]: Changed pullSecret from string to bool

[TobyTheHutt]

What this PR does:
The PR changes the data type of the pullSecret from string to bool.
This removes some confusion for the key, since the provided value will in no scenario influence the name of the secret.

Which issue this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close that issue when PR gets merged):
fixes #109

Notes for Reviewer:
This PR is a quick solution which does not change the behaviour of the chart itself. It is only to help prevent confusion when trying to use that value.

If desired, I will also commit an additional optional Values.secrets.data.registry.pullSecret.name value which will then generate the pullSecret with the pattern {{ template "library.name" $root }}-{{ $name }}.

Checklist:

• Pull Request title in format [chart]: Changed Something
• Updated documentation in the README.md.gotmpl file and executed helm-docs
• Chart Version bumped
• All commits are signed-off