Juju info add config option

[hmlanigan]

Description of change

Allow channels to be displayed again, real vs canned data issue. Instead of latest, the tracks that are latest are an empty string, insert "latest" for display purposes.

Add the ability to display charm config options in the juju info output. Handy to know before deploying.

QA steps

$ export JUJU_DEV_FEATURE_FLAGS="charm-hub"
$ juju bootstrap localhost testme
$ juju add-model seven --config charm-hub-url="https://api.staging.snapcraft.io"
$ juju info apitest-ubuntu-qa --config
name: apitest-ubuntu-qa
charm-id: 4HVy9HLeoMQ0eUALfsvE6ifqtL6DqUPm
summary: |
  juju-qa version of the ubuntu charm.
publisher: Heather Lanigan
supports: focal, xenial, bionic
subordinate: false
store-url: https://api.staging.snapcraft.io/v2/charms/apitest-ubuntu-qa
description: |
  juju-qa version of the ubuntu charm.
channels: |
  latest/stable:     1     (1)  250kB
  latest/candidate:  1     (1)  250kB
  latest/beta:       1     (1)  250kB
  latest/edge:       2     (2)  250kB
settings:
  status:
    type: string
    description: temporary string for unit status
    default: hello
  thing:
    type: string
    description: A thing used by the charm.
    default: "\U0001F381"

[SimonRichardson]

$$merge$$

[SimonRichardson]

$$merge$$