Introduce active attribute into Host

[manoelcampos]

Add an attribute "active" to Host to enable it to be powered on and off as desired.

Detailed information about how the feature should work

A setter should be defined to change the active status of the Host.

When a Host is set to powered off (active false), no VMs will be submitted to it
and if it's a PowerHost, no power will be consumed. While they have running VMs, they will continue to consume power.

If the Host has running Vms and it is set to powered off, it has to be simulated a scheduled shutdown, enabling all VMs to finish before the Host is shut down.