Prefer Atlas variables over local variables

[sethvargo]

This issue comes from our discussion today. The UX for terraform push with respect to variables should be the following:

• On the first terraform push, all local variables are sent and stored in Atlas

• On subsequent terraform push, only local variables that have not yet been stored in Atlas are sent

  • A warning should be displayed telling the user that only new variables are added

• A new CLI option/flag should be added that allows the user to update 1+ remote terraform variables via the command line as part of terraform push, something like:

  $ terraform push -var="num-nodes=5"

  This will always overwrite (or create) the remote terraform variable in Atlas

/cc @pearkes @mitchellh

[ghost]

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