Refactor koperator reserved label keys

[panyuenlau]

Q	A
Bug fix?	no
New feature?	no
API breaks?	no
Deprecations?	no
License	Apache 2.0

What's in this PR?

This PR refactors current implementation to replace all the hard-coded koperator reserved label keys ("app, "kafka_cr", and "brokerId")with const variables

Why?

Using hard-coded strings for the reserved label keys comes with some drawbacks:

• Kinda bad coding style
• Hard to maintain (especially for people that are not familiar with the existing implementation)
• Increased chances of introducing careless bugs
• May introduce inconsistency between koperator and projects that have koperator as a depenency

Therefore this PR aims to replace the label keys with const variables defined along with the kafkacluster resource

Checklist

• Implementation tested
• Error handling code meets the guideline
• Logging code meets the guideline
• User guide and development docs updated (if needed)

[pregnor]

LGTM

Thank you so much for this. 👍