RFE: Use "--ignore-installed" by default when global site packages is enabled

Issue #354 on hold
Nick Coghlan
created an issue

Working on the use case described in https://bitbucket.org/hpk42/tox/issues/236/tox-must-create-the-source-distribution#comment-29617000 I ran into an interestingly cryptic error message related to nosetests: tox was attempting to use the system nose installation, even though I'd explicitly specified it as a dependency to be installed into the virtual environment.

I eventually figured out the problem, and was able to workaround it by overriding install_command to add "--ignore-installed" to the default settings. That meant everything declared as a dependency would be installed into the virtual environment, even if it already existing in the system site packages, leaving the latter to be used only for the components not available in a venv friendly format.