Add virtualenv support to the pip package provider

Now that #6527 is merged into master, it is possible to easily add support for virtualenvs and fully complete PUP-1088.

A virtualenv argument could be added to the package type in order to instrument pip to install the requested package into the given virtualenv directory.

For example:

package { "my-python-package":

    ensure => latest,

    provider => pip,

    virtualenv => "/path/to/virtualenv

}

The content of the virtualenv parameter will be passed directly to the --environment parameter of the pip install command

Some references:

• http://pypi.python.org/pypi/pip
• http://www.pip-installer.org/en/latest/index.html#using-pip-with-virtualenv
• https://github.com/puppetlabs/puppet/blob/master/lib/puppet/provider/package/pip.rb