Power states as type aliases

The power states passed as parameters to the `PowerSetter` interfaces should be constants and type aliases 

https://github.com/bmc-toolbox/bmclib/blob/master/bmc/power.go#L24 

This was also suggested in https://github.com/bmc-toolbox/bmclib/pull/261#discussion_r854482624

```
type PowerState string

PowerStateOn PowerState = "On"
PowerStateOff PowerState = "Off"
```