Remove natty from distro list in packaging

Natty is end of life, so we no longer build packages for it.
Merge branch 'remove-natty'
(maint) Add f18 mocks to build_defaults

Fedora 18 has been released to the world, so we should build packages for it.
This commit adds the fedora 18 mocks to the build_defaults so we can easily
supply fedora 18 packages.