.env used during provisioning available to recipes.

This is useful for tools like foreman and runit which can use them when
configuring the environment for tools on the box. Otherwise this env
file needs to be reconstructed by recipes on the server.
Source the env file from plow.sh.

Previously the contents of this file were being cat'ed into plow.sh
which seems unneccesary.
Comment on the purpose of the env file.
Merge pull request #21 from MarkBennett/make-raw-env-available-to-plow-recipes

.env used during provisioning available to recipes.