Add missing library

According to pyflakes:

cinder/api/common.py:246: undefined name 'minidom'
cinder/api/common.py:254: undefined name 'minidom'
cinder/api/common.py:272: undefined name 'minidom'
cinder/api/common.py:284: undefined name 'minidom'

Add from xml.dom import minidom

Change-Id: Iadb1586ac15c29bb1f2154324e529976f82c19f0
Signed-off-by: Chuck Short <chuck.short@canonical.com>