Username

[bdwyertech]

Username should be optional, such that the integrated RunDeck user configuration can be utilized. The fallback currently seems to be ENV['USER'].

We can feed in property references, but unfortunately it seems there is no way to configure a sane default. I would have expected something like the below to work, but unfortunately it's a no go. Maybe I've got the interpolation syntax messed up? Who knows.
"username": "${option.ssh.user!\"default_user\"}"