Add ability to suppress nested venv courtesy notice

Added an environment variable, PIPENV_SUPPRESS_NESTED_WARNING, to be
used to suppress the courtesy notice from warn_in_virtualenv.
Changed wording of the courtesy notice to mention suppression variable.
Fixes #2527.

pipenv/core.py

@@ -1612,19 +1612,28 @@ def gen(out):
 
 
 def warn_in_virtualenv():
-    from .environments import PIPENV_USE_SYSTEM, PIPENV_VIRTUALENV
+    from .environments import (
+        PIPENV_USE_SYSTEM,
+        PIPENV_VIRTUALENV,
+        PIPENV_SUPPRESS_NESTED_WARNING,
+    )
 
     # Only warn if pipenv isn't already active.
     pipenv_active = os.environ.get("PIPENV_ACTIVE")
-    if (PIPENV_USE_SYSTEM or PIPENV_VIRTUALENV) and not pipenv_active:
+    if (
+        (PIPENV_USE_SYSTEM or PIPENV_VIRTUALENV)
+        and not (pipenv_active or PIPENV_SUPPRESS_NESTED_WARNING)
+    ):
         click.echo(
             "{0}: Pipenv found itself running within a virtual environment, "
             "so it will automatically use that environment, instead of "
             "creating its own for any project. You can set "
             "{1} to force pipenv to ignore that environment and create "
-            "its own instead.".format(
+            "its own instead. You can set {2} to suppress this "
+            "warning.".format(
                 crayons.green("Courtesy Notice"),
                 crayons.normal("PIPENV_IGNORE_VIRTUALENVS=1", bold=True),
+                crayons.normal("PIPENV_SUPPRESS_NESTED_WARNING=1", bold=True),
             ),
             err=True,
         )

pipenv/environments.py

@@ -162,6 +162,14 @@
 Default is to use the compatibility shell if possible.
 """
 
+PIPENV_SUPPRESS_NESTED_WARNING = bool(os.environ.get(
+    'PIPENV_SUPPRESS_NESTED_WARNING'))
+"""If set, suppress the courtesy notice regarding nested virtual environments.
+
+Default is to show a courtesy notice when pipenv finds itself running inside
+another virtual environment.
+"""
+
 PIPENV_TIMEOUT = int(os.environ.get("PIPENV_TIMEOUT", 120))
 """Max number of seconds Pipenv will wait for virtualenv creation to complete.
 

tests/unit/test_core.py

@@ -0,0 +1,20 @@
+import io
+
+import pytest
+import mock
+from contextlib import redirect_stderr
+
+from pipenv.core import warn_in_virtualenv
+
+
+@mock.patch('pipenv.environments.PIPENV_VIRTUALENV', 'totallyrealenv')
+@mock.patch('pipenv.environments.PIPENV_SUPPRESS_NESTED_WARNING', '1')
+@pytest.mark.core
+def test_suppress_nested_venv_warning():
+    f = io.StringIO()
+    # Capture the stderr of warn_in_virtualenv to test for the presence of the
+    # courtesy notice.
+    with redirect_stderr(f):
+        warn_in_virtualenv()
+    output = f.getvalue()
+    assert 'Courtesy Notice' not in output