Update Terraform.gitignore

Improvements:

• .terraform directories could be created in any subdirectory when user runs terraform init.
• Quite an interesting variations of`.tfstate` files are created by Terraform such as .*.tfstate.d or .*.tfstate.lock.info. It's best not to name any file with *.tfstate.* pattern.
• *.tfvars files often contain private data and not supposed to be committed.