Further improvements and savings in Jenkins pipeline (#5904)

* Publish artifacts only on the master and release branches

* Build CUDA only for Compute Capability 7.5 when building PRs

* Run all Windows jobs in a single worker image

* Build nightly XGBoost4J SNAPSHOT JARs with Scala 2.12 only

* Show skipped Python tests on Windows

* Make Graphviz optional for Python tests

* Add back C++ tests

* Unstash xgboost_cpp_tests

* Fix label to CUDA 10.1

* Install cuPy for CUDA 10.1

* Install jsonschema

* Address reviewer's feedback

tests/python/test_plotting.py

@@ -15,7 +15,7 @@ except ImportError:
     pass
 
 
-pytestmark = pytest.mark.skipif(**tm.no_matplotlib())
+pytestmark = pytest.mark.skipif(**tm.no_multiple(tm.no_matplotlib(), tm.no_graphviz()))
 
 
 dpath = 'demo/data/'

tests/python/test_with_sklearn.py

@@ -437,6 +437,7 @@ def test_sklearn_api_gblinear():
 
 
 @pytest.mark.skipif(**tm.no_matplotlib())
+@pytest.mark.skipif(**tm.no_graphviz())
 def test_sklearn_plotting():
     from sklearn.datasets import load_iris
 

tests/python/testing.py

@@ -98,6 +98,26 @@ def no_json_schema():
         return {'condition': True, 'reason': reason}
 
 
+def no_graphviz():
+    reason = 'graphviz is not installed'
+    try:
+        import graphviz  # noqa
+        return {'condition': False, 'reason': reason}
+    except ImportError:
+        return {'condition': True, 'reason': reason}
+
+
+def no_multiple(*args):
+    condition = False
+    reason = ''
+    for arg in args:
+        condition = (condition or arg['condition'])
+        if arg['condition']:
+            reason = arg['reason']
+            break
+    return {'condition': condition, 'reason': reason}
+
+
 # Contains a dataset in numpy format as well as the relevant objective and metric
 class TestDataset:
     def __init__(self, name, get_dataset, objective, metric