Make run_ansible virtualenv system_site_packages conditional
Make run_ansible virtualenv system_site_packages conditional

Make it conditional upon the edX release.

See https://github.com/mitodl/salt-ops/commit/17eaba3835af090720343977defa8715e924218e

Having system site-packages show through to the virtualenv was causing
issues with non-Juniper AMI builds, per @blarghmatey

mitodl/salt-ops,mitodl/salt-ops