Add an option to force broker to always assign the same machine to a user

[dynamiccast]

And an option in the configurator (this should not be updatable from the interface since the implications are unknown).

When a VM is about to be terminated and if this option is checked, we shut down the virtual machine instead of terminating it.

When a user requests a VM, If the VM is shutdown we start the VM prior to respondig to his request.

[dynamiccast]

Fixes in #312