refactor: Refactor ocm methods out of register cmd file

[dimakis]

to verify, One will have to walk through the rhoas dedicated register-cluster cmd.

Closes #

Verification Steps

1. Do x
2. Do y

Type of change

• [ ] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] Documentation change
• [ ] Other (please specify)

[dimakis]

A couple of suggestions, the main issue is the lack of tests 🙁

@andreaTP refactored and addresses cluster pagination, also adds an error message when there are no available valid openshift clusters to choose from