A couple of weeks ago I see an email on the smartos emailing list mentioning that John had created a terraform smartos provider for terraform.
Installing the provider is pretty easy. I did the following steps in my smartos zone:
My current go version is: go version go1.10 solaris/amd64

Make sure you have setup go correctly:

# mkdir ~/gopath
# export GOPATH=~/gopath
# go get github.com/john-terrell/terraform-provider-smartos

Lets go to the download directory and compile the source code:

# cd gopath/src/github.com/john-terrell/terraform-provider-smartos
# make build
# ls gopath/bin/terraform-provider-smartos

Now you can use it as a terraform provider. I copied the binary file into the terraform plugin dir:

# cp ~/gopath/bin/terraform-provider-smartos ~/.terraform.d/plugins

You are now good to go to follow the example in the github repo.
Kudos go to John for creating this awesome plugin!