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.

Before:
```
$ ansible -i hosts local -m easy_install -a "name=Mako virtualenv=/home/sayap/virtualenv/test1"
localhost | success >> {
    "binary": "/usr/bin/easy_install", 
    "changed": false, 
    "name": "Mako", 
    "virtualenv": "/home/sayap/virtualenv/test1"
}

$ ls ~/virtualenv/test1/lib/python2.7/site-packages/
easy_install.py  easy_install.pyc  _markerlib  pip  pip-1.4.1-py2.7.egg-info  pkg_resources.py  pkg_resources.pyc  setuptools  setuptools-0.9.8-py2.7.egg-info
```

After:
```
$ ansible -i hosts local -m easy_install -a "name=Mako virtualenv=/home/sayap/virtualenv/test2"
localhost | success >> {
    "binary": "/home/sayap/virtualenv/test2/bin/easy_install", 
    "changed": true, 
    "name": "Mako", 
    "virtualenv": "/home/sayap/virtualenv/test2"
}

$ ls ~/virtualenv/test2/lib/python2.7/site-packages/
easy-install.pth  easy_install.pyc      _markerlib                              pip                       pkg_resources.py   setuptools
easy_install.py   Mako-0.9.0-py2.7.egg  MarkupSafe-0.18-py2.7-linux-x86_64.egg  pip-1.4.1-py2.7.egg-info  pkg_resources.pyc  setuptools-0.9.8-py2.7.egg-info
```