Allow customizing Pod CIDR and Service CIDR on bare metal

[invidian]

It should be simple exposing the knob which is already used for other platforms.

Useful links for newcomers:

• Where to add fields:

  lokomotive/pkg/platform/baremetal/baremetal.go

  Line 33 in a313f02

  type config struct {

• What to add:

  lokomotive/pkg/platform/aws/aws.go

  Lines 74 to 75 in a313f02

  	PodCIDR string `hcl:"pod_cidr,optional"`
  	ServiceCIDR string `hcl:"service_cidr,optional"`

• Terraform template example:

  lokomotive/pkg/platform/aws/template.go

  Lines 49 to 54 in a313f02

  	{{- if .Config.PodCIDR }}
  	pod_cidr = "{{.Config.PodCIDR}}"
  	{{- end }}
  	{{- if .Config.ServiceCIDR }}
  	service_cidr = "{{.Config.ServiceCIDR}}"
  	{{- end }}

• Docs should be updated as well: https://github.com/kinvolk/lokomotive/blob/9065238f2c2b1e0262363eff4f1bab58ef66092c/docs/configuration-reference/platforms/baremetal.md#attribute-reference

Tests are welcome, but not requires I guess, as we don't have examples.