Changed PIPENV_SUPPRESS_NESTED_WARNING to PIPENV_VERBOSITY, an integer
value that defaults to 0 (verbose).
Changed test in test_core.py to use capsys to capture standard
output/error in order to improve compatibility with Python 2.
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.