[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/test_python.sh

@@ -0,0 +1,45 @@
+#!/bin/bash
+set -e
+set -x
+
+suite=$1
+
+# Install XGBoost Python package
+wheel_found=0
+for file in python-package/dist/*.whl
+do
+  pip install --user ${file}
+  wheel_found=1
+  break  # need just one
+done
+if [ "$wheel_found" -eq 0 ]
+then
+  pushd .
+  cd python-package
+  python setup.py install --user
+  popd
+fi
+
+# Run specified test suite
+case "$suite" in
+  gpu)
+    pytest -v -s --fulltrace -m "(not slow) and (not mgpu)" tests/python-gpu
+    ;;
+
+  mgpu)
+    pytest -v -s --fulltrace -m "(not slow) and mgpu" tests/python-gpu
+    cd tests/distributed
+    ./runtests-gpu.sh
+    ;;
+
+  cpu)
+    pytest -v -s --fulltrace tests/python
+    cd tests/distributed
+    ./runtests.sh
+    ;;
+
+  *)
+    echo "Usage: $0 {gpu|mgpu|cpu}"
+    exit 1
+    ;;
+esac