Add --list-available-roles

[AlexeyRaga]

Currently gsts interactively displays a list of roles to chose from when --aws-role-arn is not provided.

For automation, it would be very helpful to get this list in non-interactive way with a new --list-available-roles flag.

This flag could output the list of roles and let gsts exit.

Obeying --json would be a huge plus :)

[ruimarinho]

Hey @AlexeyRaga -- I'm ok with this. I don't know when I'll be able to work on it but perhaps someone will submit a PR in the meantime.