Allow VCS plugins to provide a configuration tree

[strauss]

Currently, a BCFG2 repository can have uncommitted changes that get deployed to clients. I've updated the Bzr plugin to note this in snapshots by appending a plus sign to the revision number to indicate the application of uncommitted changes. But this is not an ideal solution.

Ideally, version control plugins should be able to provide BCFG2 with a working tree to use for configuration purposes. This would allow plugins like Bzr to provide a pure revision, even if it's pulled from /var/lib/bcfg2. This would instill better discipline in systems management by requiring a commit to deploy a change in configuration.