Support --format=json for attach/detach/disable/enable

[robert-ancell]

When using the ua client from a desktop environment it is desirable to get back information on requests in a form that can be easily parsed. The proposal is to add a --format=json flag form for these much like the status command does.

For example, currently:

$ sudo ua enable foo
To use 'foo' you need an Ubuntu Advantage subscription
Personal and community subscriptions are available at no charge
See https://ubuntu.com/advantage

proposed:

$ sudo ua enable foo --format=json
{"result": "error", "error-code": "no-subscription", "error-text": "To use 'foo' you need an Ubuntu Advantage subscription\nPersonal and community subscriptions are available at no charge\nSee https://ubuntu.com/advantage"}

[robert-ancell]

See #1814 for showing progress information, which may be solved at the same time.

[orndorffgrant]

This was addressed over several PRs including #1912 #1949 #1953