return the name of the cache file created
return the name of the cache file created

Have the function that creates the cache file return the name of the
file created so the caller doesn't have to know how that name is
constructed from default values.

Change-Id: I576045e1591badc84722e28072c7aed36a35be38
Signed-off-by: Doug Hellmann <0798ebecfb7bc4dfb9d0445c2fa030eb64e1fa1c@doughellmann.com>

openstack/reno