Add sudo_user env var

[bitprophet]

I.e. allow block-level or global modification of which user sudo executes as.

Right now, users who always need to run sudo as another user (e.g. to run commands "as" a user which does not have SSH login privileges, and which is not root, such as daemon user accounts) have no useful recourse.

Easiest solution is just a new env var that acts as a default if the kwarg is not given.