BugTask target key attributes should not be directly writable

[William Grant]

Security declarations presently permit direct setting of BugTask.{distribution,distroseries,product,productseries,sourcepackagename}. But additional rules and restrictions apply to these, so all writes should go through transitionToTarget.

Most of this was fixed in my LaunchpadTargetWidget-on-BugTaskEditView series, so this should be mostly just changing the security declarations.

[Launchpad QA Bot (lpqabot)]

Fixed in stable r13750 (http://bazaar.launchpad.net/~launchpad-pqm/launchpad/stable/revision/13750) by a commit, but not testable.