Build/vendor boost and yaml-cpp in the SLES 15 puppet-agent package

Apparently we don't build/ship boost and yaml-cpp on sles 15: https://github.com/puppetlabs/puppet-runtime/blob/master/configs/projects/agent-runtime-master.rb#L37

Given that we build/vendor these on every other platform, we should do the same for SLES 15 so we have a unified behavior on all of our supported agent platforms.