Allow tsh to go background and without executing remote command

[bric3]

What happened:
tsh don't have the options to make a command just for port forwarding.

What you expected to happen:
When running a port forwarding like tsh ssh -L localhost:8000:internal-name:80 dev-role@node, I would like tsh to have equivalent ssh

• -f to go background after login
• -N to avoid the execution of a remote command

In other words I would like this command to just forward ports.

How to reproduce it (as minimally and precisely as possible):
I think the above command is enough.

Environment:

• Teleport version (use teleport version): Teleport v2.7.4
• Tsh version (use tsh version): Teleport v2.7.4
• OS (e.g. from /etc/os-release):

  ProductName:	Mac OS X
  ProductVersion:	10.14
  BuildVersion:	18A391
  

Browser environment

• Browser Version (for UI-related issues): N/A
• Install tools: brew
• Others: N/A

[kontsevoy]

@bric3 love this suggestion! we'll schedule this to a future release

[klizhentas]

@kontsevoy let's move this out of Pleasanton, we don't have enough time to roll this out

[bric3]

Arg too bad. Thx for considering this though ;)

[kontsevoy]

@klizhentas let's not forget. There's a commercial customer who asked for this as well.

[klizhentas]

let's do 4.1 release milestone and add this and other improvements there.

[webvictim]

I have another potential customer also requesting this.

[klizhentas]

when it comes to -f I'm not sure we should be trying to implement it due to complexity of a go runtime:

golang/go#227

would it be OK to have just -N and then folks can shell features like & and nohup + io forwarding for the rest?