Handling validation in conductor consistently

Change where we do validation in change_node_power_state to follow
where we do it for do_node_deploy and vendor_passthrough. That
is, we perform validation synchronously before we spawn off a
thread to perform the longer running action.

Change-Id: Ic1f791bb5ef49c033e4baff8d3f6900091dbf332
Closes-bug: #1294676
Merge "Handling validation in conductor consistently"