Log the value of OS_CONFIG_FILES
Log the value of OS_CONFIG_FILES

We pass the list of json files containing the collected metadata to
os-refresh-config using the OS_CONFIG_FILES env variable, so it's a
pretty useful piece of information to log.

Change-Id: Id09e3f352e6a5a09e4183c0743a6e99a2783a888

openstack/os-collect-config