Validate mac address on Hardware spec: (#628)

## Description


This only allows lower case mac addresses. This simplifies matching/looking up mac addresses. Without this values could be upper case or mixed case. Upper case and mixed cases will cause clients to miss lookups if using the wrong case. This standardized to lowercase to remove this error prone interaction.

## Why is this needed



Fixes: #

## How Has This Been Tested?





## How are existing users impacted? What migration steps/scripts do we need?





## Checklist:

I have:

- [ ] updated the documentation and/or roadmap (if required)
- [ ] added unit or e2e tests
- [ ] provided instructions on how to upgrade

config/crd/bases/tinkerbell.org_hardware.yaml

@@ -108,6 +108,7 @@ spec:
                             format: int64
                             type: integer
                           mac:
+                            pattern: ([0-9a-f]{2}[:]){5}([0-9a-f]{2})
                             type: string
                           name_servers:
                             items:

pkg/apis/core/v1alpha1/hardware_types.go

@@ -115,6 +115,7 @@ type OSIE struct {
 
 // DHCP configuration.
 type DHCP struct {
+	// +kubebuilder:validation:Pattern="([0-9a-f]{2}[:]){5}([0-9a-f]{2})"
 	MAC         string   `json:"mac,omitempty"`
 	Hostname    string   `json:"hostname,omitempty"`
 	LeaseTime   int64    `json:"lease_time,omitempty"`