apt-ftparchive*.conf: don't hard code Ubuntu version

Use the UBUNTU_RELEASE and (the newly introduced) UBUNTU_RELEASE_NUMBER
variables instead.

The idea is to have a single place (config.mk) where the Ubuntu (kernel,
whatever) versions are defined (instead of spreading that data over 10+ files)

Change-Id: If6b298aa51cdffde6c633c3b7128173148fe95aa
Related-bug: #1383641
Merge "apt-ftparchive*.conf: don't hard code Ubuntu version"