command: Restore support for terraform.tfvars.json
command: Restore support for terraform.tfvars.json

In the new implementation of collecting variables I initially forgot the
JSON variant of terraform.tfvars.

This fix is verified by TestApply_varFileDefaultJSON now passing.

nlamirault/terraform,mattray/terraform,VladRassokhin/terraform,drebes/terraform,pdecat/terraform,heimweh/terraform,drebes/terraform,heimweh/terraform,iljaskevic/terraform,nlamirault/terraform,pradeepbhadani/terraform,ewbankkit/terraform,pryorda/terraform,apparentlymart/terraform,VladRassokhin/terraform,apparentlymart/terraform,iljaskevic/terraform,gechr/terraform,pradeepbhadani/terraform,luis-silva/terraform,pdecat/terraform,youhong316/terraform,luis-silva/terraform,tommynsong/terraform,gechr/terraform,nlamirault/terraform,rnaveiras/terraform,pdecat/terraform,tpounds/terraform,brandontosch/terraform,heimweh/terraform,tpounds/terraform,apparentlymart/terraform,tpounds/terraform,ewbankkit/terraform,rnaveiras/terraform,pdecat/terraform,heimweh/terraform,pryorda/terraform,ewbankkit/terraform,VladRassokhin/terraform,hashicorp/terraform,rnaveiras/terraform,mattray/terraform,brandontosch/terraform,youhong316/terraform,tpounds/terraform,brandontosch/terraform,pryorda/terraform,youhong316/terraform,brandontosch/terraform,hashicorp/terraform,luis-silva/terraform,heimweh/terraform,VladRassokhin/terraform,hashicorp/terraform,luis-silva/terraform,mattray/terraform,gechr/terraform,iljaskevic/terraform,iljaskevic/terraform,youhong316/terraform,drebes/terraform,tommynsong/terraform,tommynsong/terraform,apparentlymart/terraform,brandontosch/terraform,mattray/terraform,ewbankkit/terraform,drebes/terraform,pradeepbhadani/terraform,apparentlymart/terraform