Terraform v0.12 support

[stefansundin]

A proof-of-concept. Fixes #11.

See:

• Terraform 0.12 support #11
• https://discuss.hashicorp.com/t/provider-plugins-that-live-for-the-duration-of-a-terraform-run/2262

Due to changes in the plugin lifecycle in Terraform v0.12, things get a lot more complicated. To work around that, when the provider is started, it launches a copy of itself that sticks around until the main terraform process has exited. The copy starts a gRPC server that accepts connections from the ssh_tunnel data sources, and opens and keeps the SSH tunnels alive.

I haven't done thorough testing yet, but it seems to work great. There are a few TODOs in there as well. Please let me know what you think!

[mirnujAtom]

Hey @stefansundin, it's so great to hear that the project is alive!
Will definitely test it our and provide feedback :)

[mirnujAtom]

Hey @stefansundin
Just tested the provider with tf12, Great Job!

The only issue I had is that looks like there is no value for Port returned from "client.grpcClient.OpenTunnel" on line 110, so I had to cut the port from local_address (EffectiveAddress) to be able to use a random port in TF:

               if resp.EffectiveAddress != localAddress {
			log.Printf("[DEBUG] localAddress: %v", resp.EffectiveAddress)
			d.Set("local_address", resp.EffectiveAddress)
		}
		d.Set("port", resp.Port)

//Dirty fix goes here --> 
		la := d.Get("local_address").(string)
		lc := strings.LastIndex(la,":")
		d.Set("port", la[lc+1:len(la)])

[stefansundin]

I noticed that bug as well, but I didn't manage to track it down. I will try to take another look soon.