Add JSON output option

[dboreham]

For situations where machine is run by other software, to make it easier to parse its output.
Add an option that causes any output to be JSON formatted.
Initial implementation should target the list subcommand.

[dboreham]

It looks like JSON output for the list subcommand is already implemented:

• machine/subcommands/list.py defines a --output / -o option (line 63)
• When --output json is passed, it calls print_json which serializes VMs via vm_to_json_obj and outputs JSON (lines 20-21, 85)
• vm_to_json_obj in machine/util.py produces a dict with id, name, tags, region, ip, and type

Usage: machine list -o json

Should this issue be closed, or is there additional work needed (e.g., extending JSON output to other subcommands, adding --output as a global option, validation/choices on the --output flag)?

[dboreham]

Ah ok, I hadn't noticed that feature existed. Perhaps we could add the following:

1. An example of JSON output in the docs (for the subcommands where the existence of the JSON output option is documented but not the format of said output).
2. JSON as an output option for the following subcommands: create, projects, domains, types.