Add TFE_* env vars as default values for options

[skierkowski]

Problem: There are a lot of fields that are set as options on the command line but are likely to be reused frequently.

Solution: Provide environment variables to set default values for the options. So each option should first accept the field provided in the option and if one is not provided look up the TFE_ value from the environment variables.

Example: The --token option is used to set the token, but you can also set it using TERRAFORM_ENTERPRISE_TOKEN.