Source environment variables from `.env` file when adding deployment secrets

[david-castaneda]

Currently, setting secrets is a very manual process and can be streamlined using a users .env file. The .env file can be referenced in the command line so that they get sourced and uploaded as secrets.

$ krane secrets add my-deployment -f ./.env

If an environment variable defined in the .env ALREADY exists it should not be overridden

Done uploading secrets for my-deployment sourced from ./env:
DB_URL: @DB_URL
DB_USER: @DB_USER

We found some secrets which where already in use and not overriden:
DB_PASSWORD: @DB_PASSWORD