"triton ssh" (w/ no arguments) to open ssh if only one running instance present?

[curiositycasualty]

This is probably a pattern that could work for several of the subcommands: If only a single running instance exists, assume that the user is referring to that.

Currently, most subcommands require your explicitely define the SHORTID of that one instance you want. Let me illustrate:

$ triton ls -H
a34f4174  packer-builder-singel-ubuntu-16.04  ubuntu-certified-16.04@20180109  running  K      14m

$ triton ssh
triton instance ssh: error (Usage): missing INST arg
usage: triton instance ssh [-h] [USER@]INST [SSH-ARGUMENTS]

$ triton ssh a34f4174
Welcome to Ubuntu 16.04.3 LTS (GNU/Linux 4.4.0-108-generic x86_64)
Certified Ubuntu Cloud Image
<snip>
ubuntu@a34f4174-f330-e929-d12e-cfc3705dfb0d:~$

I'm proposing that it work more like this:

 $ triton ls -H
a34f4174  packer-builder-singel-ubuntu-16.04  ubuntu-certified-16.04@20180109  running  K      14m

 $ triton ssh
Welcome to Ubuntu 16.04.3 LTS (GNU/Linux 4.4.0-108-generic x86_64)
Certified Ubuntu Cloud Image
<snip>
ubuntu@a34f4174-f330-e929-d12e-cfc3705dfb0d:~$