Refactor how PARTNER_ variable are handled.
Refactor how PARTNER_ variable are handled.

This cleans up how partner-specific variables get added to the default environment with the PARTNER_ prefix, so they can be accessed globally.
It fixed a bug where every time partners were switched, you got partner_partner_partner_blah, and also actually cleans the default environment instead of just setting the values to ""
Also, makes sure overrides from the commandline to partner sections are correctly reflected.

preed/quickrelease