jenkins: use in-tree clone for the ceph-build repo

Historically we have had a single system-wide clone of the ceph-build repository on each builder, at `/srv/ceph-build`.

The disadvantage to this configuration is that there is no way to ensure that each of these clones are up to date. When we make changes to the ceph-build repository on GitHub, we have to manually pull the changes into each builder's local clone at `/srv`.

Going forward, we will use the Multi-SCM plugin to Jenkins to make the ceph-setup Jenkins job use an up-to-date clone of the ceph-build repository. This will ensure that Jenkins is using the very latest code from ceph-build.

The ceph-build repository will be cloned into the root of the tree. Remove the references to `/srv/ceph-build` and just use the relative path, "`ceph-build`".