[feature request] Make variable source pluggable

[akamensky]

Currently variable need to be provided either on CLI or as a file. This may be a limiting factor to the uses of this tool. I think being able to have multiple sources for variables can be very beneficial to this tool.

As an example:

1. Pack that defines variables A, B and C
2. Consul KV has key value pairs for B and C
3. Running the pack with variables A and B and also pointing it to Consul KV as another variable source
4. The output will use A and B as provided from CLI and C as available in Consul KV (B on CLI considered an override)

As an alternative -- extend templating engine to be able to query some of other Hashicorp tools for the data, such as Consul KV, Nomad Variables and Vault secrets engines.

I think this request also supersedes #409 with a bit of higher-level/wider scope.

[akamensky]

FWIW, for Consul KV I could now call consul-template to generate variable file from KV data and then use that file as an input for nomad-pack. But I think providing ability to directly externalize source of variables would play well with #472