Backport cb9cf61b from master to fix feature flag tear down test break on windows

[davecheney]

Fixes LP 1566303 (for real this time)

Backport cb9cf61 from master to fix feature flag tear down test break on windows.

The underlying cause is the feature tests run in parallel, and all try to mutate one registry key. The logic is roughly

• program A creates registry key (as it doesn't exist), and records the fact that it must delete the registry key.
• program B trys to create a registry key, finds that it already exists, so it records the old value, then updates it, and makes a not to restore the old value.
• program A deletes the registry key, as when it started the key did not exist, so it's behaviour is to remove the key.
• Program b trys to restore the state of the existing key, and finds the key deleted.
• Test fails.

This pattern is unsafe, we have to rewrite it to not use a singleton registry key. And apparently Tim did this on master, so backport that fix.

(Review request: http://reviews.vapour.ws/r/4505/)

[davecheney]

$$JFDI$$

[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/7281

[davecheney]

$$JFDI$$

[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/7301

[bz2]

$$fixes-1566303$$

[jujubot]

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