TagPlugin - support both terraform 0.11 and 0.12

[kmanning]

• Is the -var different between 11 and 12?
• the plugin uses -var="key=value", but the equal sign might be causing problems with v0.11.
• Either remove the = (assuming that works in 12), or build this difference in TerraformVersionPlugin.

[kmanning]

See terraform docs

• https://www.terraform.io/docs/configuration-0-11/variables.html
  • terraform apply -var 'foo={quux="bar"}' -var 'foo={bar="baz"}'
• https://www.terraform.io/docs/configuration/variables.html#variables-on-the-command-line
  • terraform apply -var='image_id_map={"us-east-1":"ami-abc123","us-east-2":"ami-def456"}'

Note that there are 2 differences:

1. 11: -var 'key=..., 12: -var='key=...
2. 11: map={key="value"}, 12: map={"key":"value"}