Remove el-5 support and other bitrot in beaker CI

This is blocking Beaker CI runs.

The el-5 redhat variants are past end of life and are unsupported. el-6 will reach EOL in 2020, as well. I think it makes sense to keep el-6 support for now, but el-5 shouldn't be used anywhere anymore. I was prompted to remove the el-5 support because it appears that the EPEL mirrors are being refactored away and are likely going to disappear soon, which makes it difficult to beaker to support the platform without hosting packages ourselves, which we are not likely going to do.