Implement partial working copies

[blee]

confman should allow the user to use partial working copies (i.e. containing only modules pertaining to the host). This is important for working copies not exported via NFS or otherwise. This should be a configurable option in confman.conf.

This is slightly problematic because Subversion does not natively support the notion of partial working copies. However, as long as users follow the abstraction (i.e. use 'confman update'), this should not be a problem.

This may make it less intuitive to add modules to a host's recipe because those modules will not be readily visible in the working copy. This limitation may be mitigated by some sort of module-listing interface.