Add ability to use Passenger Enterprise

[shaneog]

Passenger Enterprise uses a different repository which requires authentication, and a license file that needs to be installed on the server.

This commit adds two new attributes, enterprise_token and enterprise_license which need to be set to install Passenger Enterprise.

Versions 4 and 5 of Passenger are still supported for both OSS and Enterprise.

[mrdziuban]

Any movement on this? I'm interested in taking advantage of the changes here.