Set packer cli options from environment variables

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#### Description

I would like to be able to set options for packer commands from environment variables. I realize this is possible for input variables, using `PKR_VAR_`. But it does not seem possible for arguments to the cli itself, such as `-only` and `-except`. For example, terraform allows this using [TF_CLI_ARGS and TF_CLI_ARGS_name](https://www.terraform.io/cli/config/environment-variables#tf_cli_args-and-tf_cli_args_name).

Generally, I would expect the env value to be used as a default, and args provided directly on the cli to override the same option from the env.

#### Potential References

* [Terraform TF_CLI_ARGS and TF_CLI_ARGS_name](https://www.terraform.io/cli/config/environment-variables#tf_cli_args-and-tf_cli_args_name)

The issue names no files or tests; begin at Packer's CLI command parsing for -only and -except and compare the referenced Terraform TF_CLI_ARGS behavior. Done means environment variables provide defaults for command options while explicitly supplied CLI arguments override the same option, with coverage for both forms.

Written by the indexing model from the issue text.