(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.
Merge pull request #441 from haus/maint/master/add_f18_mocks_to_build_defaults

(maint) Add f18 mocks to build_defaults