DPDK: Refactor rdma-core source build, allow arbitrary package build

[mcgov]

Fulfilling an ask to be able to test multiple rdma-core versions to allow development and regression tests. Refactors the rdma-core bits into their own own class.

Setting rdma_core_source to either a git or tar.gz link allows you to build from source. For git either set rdma_core_ref or it will pick the latest version sorted tag.