Nova Ironic driver get_info() to return memory stats in KBytes

According to the base class 'max_mem' and 'mem' should be returned in
KBytes, other drivers are also converting those values to KBytes so
Ironic should as well.

This patch also add the missing docstring for the get_info() method.

Change-Id: Ia7063b994d506b511271920678868d336a776ec9
Merge "Nova Ironic driver get_info() to return memory stats in KBytes"