Replace deprecated readfp method with read_file
Replace deprecated readfp method with read_file

The readfp method has been deprecated since version 3.2 [1].

[1] https://docs.python.org/3/library/configparser.html?highlight=deprecated#configparser.ConfigParser.readfp

Change-Id: Ib1aa6bba91e331f25c65a5905a89de306faa1d5a

openstack/heat-cfntools