Locate easy_install path after (optionally) creating virtualenv.

Without this, the first call to the easy_install module with a new virtualenv
will only create the virtualenv without installing the intended package, since
the `_is_package_installed` check will succeed as running /usr/bin/easy_install
as non-root user will return permission denied error with empty stdout.
Merge pull request #4075 from sayap/easy_install

Locate easy_install path after (optionally) creating virtualenv.