Terraform plugin updates to support multiple tf version

[kaufers]

The CLI semantics of terraform changed between v0.9.x and v0.10.x+. In order to handle updates this PR does the following:

• Moves all external terraform command execs to a new terraform.go file and creates a common tf interface. A terraformBase stuct implements the interface for terraform v0.9.x and terraformV10 struct implements the interface for terraform v0.10.x+ (currently supports v0.10.x and v0.11.x).
• Updates the UT flow to create a FakeTerraform struct that implements the tf interface so that we can simply implement the interface functions that are needed for testing
• Adds support in v0.10.x for the new -auto-approve=true CLI arg on terraform apply
• Adds support in v0.10.x to create the required tf.json file before invoking terraform import

[codecov[bot]]

Codecov Report

:exclamation: No coverage uploaded for pull request base (master@ce3e073). Click here to learn what that means. The diff coverage is 9%.

@@            Coverage Diff            @@
##             master     #878   +/-   ##
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  Coverage          ?   48.55%           
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  Files             ?       94           
  Lines             ?     8448           
  Branches          ?        0           
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  Hits              ?     4102           
  Misses            ?     3980           
  Partials          ?      366

Impacted Files	Coverage Δ
pkg/provider/terraform/instance/show.go	100% <ø> (ø)
pkg/provider/terraform/instance/terraform.go	0% <0%> (ø)
pkg/provider/terraform/instance/apply.go	75.27% <100%> (ø)
pkg/provider/terraform/instance/plugin.go	88.36% <62.5%> (ø)

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