[CI] Potpourri of tasks for the CI server (xgboost-ci.net)

[hcho3]

• Install security updates to Jenkins and other software packages
• For GPU test agents, use G4 instance type instead of current P2 instance type. Compared to P2, G4 is more cost efficient and uses newer microarchitecture (Turing).
• With the above, we can now add tests for cuDF. (Set up test for cudf. #4792)
• Upgrade Visual Studio 2017 to update 15.7, to get full support for C++11.
• Store nightly build artifacts in a S3 bucket with organized names (Nightly releases #4813, [jvm-packages] Jenkins CI should build snapshot JARs #4752)
• Fix SSL connection problem [CI] Jenkin CI server (xgboost-ci.net) down for emergency maintenance #4921

Feel free to make more suggestions.
@dmlc/xgboost-committer @trivialfis @rongou @sriramch

[hcho3]

As part of #4921, I upgraded GPU test agents to use G4 instance types. I only did it for the agents running Linux; I'll come back and update Windows agents later.