Add/Update docstrings in the Nova Ironic Driver

Add and update the docstrings in the Nova Ironic Driver. The 'node'
parameter from the power_on and power_off method was removed because
it's not part of the interface and wasn't being used.

To maintain the function signature consistent with the base class the
'node' parameter from the get_available_resource() method was renamed to
'nodename'.

Also, the method manage_image_cache() was removed from the driver because
it was simply calling "pass", just like the base class.

Change-Id: I0d2fe07992bde7bf09df934282b0551a35e99c1c
Merge "Add/Update docstrings in the Nova Ironic Driver"