add packer variable files option

[GwonsooLee]

Issue #, if available:

Description of changes: Add packer variable file environment variables for users to specify the custom variable files for packer. Users now can only set variables dynamically by setting PACKER_VARIABLES. If users want to modify few more variables which are not listed int PACKER_VARIABLES, then it would be helpful for them to define variables in json file and specify the file name to PACKER_VAR_FILE

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[cartermckinnon]

Looks fine to me; if you resolve the conflict and rename this PACKER_VARIABLE_FILE, I'm on board :+1:

[cartermckinnon]

This will be resolved by #1079 .