Add support for machine-only mode to skip VultrCluster dependency

[huseyinbabal]

Description

This PR introduces a "machine-only" mode that allows VultrMachine resources to be reconciled independently without requiring a VultrCluster to be present. The feature is controlled by the VULTR_MACHINE_ONLY environment variable.

Key Changes:

• Added MachineOnly() helper function in cloud/scope/common.go that checks the VULTR_MACHINE_ONLY environment variable
• Modified VultrMachine controller reconciliation logic to conditionally skip VultrCluster dependency checks when in machine-only mode
• Updated instance creation to use the machine's region specification directly instead of cluster scope
• Maintains full backwards compatibility - existing deployments continue to work unchanged

Use Cases:

• Standalone VultrMachine management without cluster-level infrastructure
• Testing and development scenarios where cluster dependency is not needed
• Simplified deployment workflows for single-machine setups

Related Issues

Related to #69 - Add support for machine-only mode to skip VultrCluster dependency

Checklist:

• Have you checked to ensure there aren't other open ../../../pulls for the same update/change?
• Have you linted your code locally prior to submission?
• Have you successfully ran tests with your changes locally?