(MCO-366) 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 we don't test
against sid but release it and we build packages for debian-testing.
Merge pull request #201 from clantant/MCO-366/master/sid

(MCO-366) Remove Sid/Unstable from build defaults