[DEV] Allow setting certain arguments with environment variables

[davidallendj]

Short Description
Some variables should be able to be set using environment variables. It might also make sense to use existing variables if they're set elsewhere (like SMD_HOST for example).

Definition of Done
This is done when environment variables can be passed to magellan

[davidallendj]

Update: This is partially implemented with the introduction of spf31/viper to handle configs. The environment variables are created by calling the viper.AutomaticEnv function. The documentation needs to be updated to include how the variables can be set.