(maint) Remove rc_mocks from build_defaults

Packaging internals have been updated to no longer require rc_mocks, so this
commit removes the rc_mocks from the build_defaults.
(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 #1496 from haus/maint/3.1.x/add_f18_mocks_to_build_defaults

Maint/3.1.x/add f18 mocks to build defaults