Revert "Update used Fedora images to version 21"

This breaks check-heat-dsvm-functional-mysql which assumes the previous glance image name.

This reverts commit 21dbe993348b794a1b77c4f9db0081d1cc32138c.

Change-Id: I77749f3f9f1a64719447ddd25ee95bc6d3afa5b3
Merge "Revert "Update used Fedora images to version 21""