cluster: introduce Machine.Inactive flag #38

reflected on disk as none on a file named region

these machines won't be included in the regional or zonal aliases

see comments

@karasz it's currently called Inactive to deal with the zero value. Active by default, otherwise indicated.

Machine.Active() renamed to Machine.IsActive()

LGTM

 		for _, r := range data {
 			switch r {
 			case "none":
 				m.Inactive = true

 	ID   int
 	Name string `json:"-" yaml:"-"`
 	Inactive        bool         `json:"inactive,omitempty"     yaml:"inactive,omitempty"`

 func (m *Machine) Active() bool {
 	return !m.Inactive
 }

 		z.ForEachMachine(func(p *Machine) bool {
 			term = fn(p)
 			if p.Active() {