state: Remove user permissions when removing model docs

[mjs]

Documents in the permissions collection were being left behind after a model was removed. This was preventing models from being migrated away from a controller and then back again.

Fixes LP 1612500.

QA Steps

$ juju bootstrap B lxd --upload-tools &
$ juju bootstrap A lxd --upload-tools &
$ juju wait
$ juju switch A
$ juju add-model foo
$ juju deploy ubuntu
# wait...
$ JUJU_DEV_FEATURE_FLAGS=migration juju migrate foo B
# wait...
$ juju switch B:foo
$ JUJU_DEV_FEATURE_FLAGS=migration juju migrate foo A
# wait...
$ juju switch A:foo
$ juju status
# ensure that agents are happy

Previously, the second migration would fail.

(Review request: http://reviews.vapour.ws/r/5432/)

[mjs]

$$merge$$

[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/8758

[mjs]

$$merge$$

[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/8759

[mjs]

$$oh-ffs$$

[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/8765

[mjs]

$$merge$$

[jujubot]

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

[jujubot]

Build failed: Does not match ['fixes-1613444']
build url: http://juju-ci.vapour.ws:8080/job/github-merge-juju/8781

[mjs]

$$merge$$

[jujubot]

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