(FACT-606) Remove Sid/Unstable from build defaults

This commit goes through the build_defaults.yaml and removes the sid and
unstable entries. Part of the reason for this removal is the lack of
testing the build but still shipping the packages and we already
build against Debian tests.
Merge pull request #714 from clantant/FACT-606/facter-2/sid

(FACT-606) Remove Sid/Unstable from build defaults