[CI] Refactor Jenkins CI pipeline + migrate all Linux tests to Jenkins (#4401)

* All Linux tests are now in Jenkins CI
* Tests are now de-coupled from builds. We can now build XGBoost with one version of CUDA/JDK and test it with another version of CUDA/JDK
* Builds (compilation) are significantly faster because 1) They use C5 instances with faster CPU cores; and 2) build environment setup is cached using Docker containers

tests/ci_build/build_via_cmake.sh

@@ -16,5 +16,5 @@ mkdir build
 cd build
 cmake .. "$@" -DGOOGLE_TEST=ON -DGTEST_ROOT=$PWD/../gtest -DCMAKE_VERBOSE_MAKEFILE=ON
 make clean
-make -j
+make -j$(nproc)
 cd ..