Fix seamicro power.validate() method definition

* Add task param to the power.validate method definition
* Also add unit tests for the power.validate method

This was not caught when unit testing Ironic using python-seamicroclient
because there is no unit test cover for this method.

Change-Id: I9ed175cffa29b8c10a32c7e121525142e4b170a5
Closes-Bug: #1303670
Merge "Fix seamicro power.validate() method definition"