[FEATURE] Leverage Secret resources for BMC credentials

[starbops]

Is your feature request related to a problem? Please describe.

The username and password of the BMC are currently stored in the KubeBMC object:

// KubeBMCSpec defines the desired state of KubeBMC
type KubeBMCSpec struct {
	// INSERT ADDITIONAL SPEC FIELDS - desired state of cluster
	// Important: Run "make" to regenerate code after modifying this file

	// To authenticate who the user is.
	// +optional
	Username string `json:"username,omitempty"`

	// The credential part of the IPMI service
	// +optional
	Password string `json:"password,omitempty"`

	// The namespace where the virtual machine is in
	VirtualMachineNamespace string `json:"vmNamespace"`

	// The actual virtual machine that this BMC controls
	VirtualMachineName string `json:"vmName"`
}

It would be great if we dropped the credential part from KubeBMC CRD and made it associated with a Secret object.

Describe the solution you'd like

Put the credential part into Secret objects and associate KubeBMC objects with them.

Describe alternatives you've considered

N/A

Additional context

N/A