Reuse validate_instance_and_node() Nova ironic Driver

The driver already counts with a function called
validate_instance_and_node() that checks which node is associated to
a given instance and raises InstanceNotFound if the instance UUID is
not found in Ironic. The same functionality was being duplicate in many
other parts of the driver, this patch simply update those parts to use
the validate_instance_and_node() function.

Change-Id: I9a2f90c7cdd0fdfcc00d49a49581c61e4f073888
Merge "Reuse validate_instance_and_node() Nova ironic Driver"