[CLI] static command to ssh into VM

[barbieri]

I was reading some examples that used gcloud and remembered about 2 very helpful commands we should try to incorporate into our CLI:

gcloud compute ssh $ID
gcloud compute scp ... files ... $ID

We could add a CLI-local set of static commands and overlay them onto sdk.Group() with NewMergeGroup() -- The issue would be how to make the interface more usable... right now we can't get general options and argv with core.Executor()...

As this is pure-CLI, we do not need to constrain ourselves to sdk.Group() and core.Executor(), but we'd need to change DynamicLoadCommand() and/or AddGroup() to handle more than the sdk.Grouper...

Suggested command:

cli virtual-machines instances ssh $ID
cli virtual-machines instances scp ... files ... $ID