provider/lxd: add interactive auth-type

[axw]

Description of change

Add the "interactive" auth-type for LXD, which is
used in add-credential for interactively adding a
credential for a LXD cloud. Currently we only
support generating credentials for local LXD;
later we will extend this to support generating
credentials for remote, untrusted LXD by prompting
the user to verify the certificate fingerprint and
enter a trust password.

QA steps

1. juju add-credential localhost

Enter credential name: foo
Auth Types
interactive*
certificate

Select auth-type: 
Loaded client cert/key from "/home/andrew/.config/lxc"
Credentials added for cloud localhost.

2. juju bootstrap localhost
3. juju add-user bob
4. juju grant bob add-model
5. lxc launch ubuntu-xenial x
6. lxc file push `which juju` x/tmp/juju
7. lxc exec x /tmp/juju register ...
8. lxc exec x /tmp/juju add-model foo

ERROR cannot auto-generate credential for remote LXD

Until support is added for verifying and authenticating to remote LXD hosts,
you must generate the credential by hand, adding the certificate to LXD using
the "lxc config trust" command.

9. juju credentials --format=yaml localhost > /tmp/localhost-credentials.yaml
10. lxc file push /tmp/localhost-credentials.yaml x/root/.local/share/juju/credentials.yaml
11. lxc exec x /tmp/juju add-model foo

Uploading credential 'localhost/bob/foo' to controller
Added 'foo' model on localhost/localhost with credential 'foo' for user 'bob'

Documentation changes

There is a change in workflow, but probably unusual enough that it doesn't need documenting?

Bug reference

Does not fix any bugs, but hopefully alleviates some of the pain caused by https://bugs.launchpad.net/juju/+bug/1662587.

[wallyworld]

Can we point the user to online doc here, just like we do in other cases, eg how to properly configure lxd kernel params.
The text tells to user something needs to be done but doesn't show them how etc. Online we can walk them through the steps. When to run autoload-credential? When to choose interactive vs certificate etc. Describe the main usage scenarios.

[axw]

juju/docs#1636

[axw]

I've added a pointer to the LXD page on jujucharms.com.

[axw]

Additional QA step:

1. juju autoload-credentials, select LXD/localhost
2. Confirm there's a "server-cert" attribute in the credential, and that bootstrap succeeds.
3. sudo service lxd stop
4. juju autoload-credentials, observe that there's no LXD/localhost option

[axw]

$$merge$$

[jujubot]

Status: merge request accepted. Url: http://juju-ci.vapour.ws:8080/job/github-merge-juju

[axw]

$$merge$$

[jujubot]

Build failed: Generating tarball failed
build url: http://juju-ci.vapour.ws:8080/job/github-merge-juju/10251

[axw]

$$merge$$

[jujubot]

Status: merge request accepted. Url: http://juju-ci.vapour.ws:8080/job/github-merge-juju

[jujubot]

Build failed: Tests failed
build url: http://juju-ci.vapour.ws:8080/job/github-merge-juju/10252

[axw]

$$merge$$

[jujubot]

Status: merge request accepted. Url: http://juju-ci.vapour.ws:8080/job/github-merge-juju