Remove "node" parameter from methods handling power state in docs

This patch fixes the doc about methods handling power state
to remove the parameter "node".
* validate
* get_power_state
* set_power_state
* reboot

Change-Id: I5c0425e1502f304b62566aa1e63823214888ef66
Partial-Bug: #1312632
Merge "Remove "node" parameter from methods handling power state in docs"