confwiz: set a convergence domain based on root_dir upon config

confwiz: set a convergence domain based on root_dir upon config

when the confwiz configures a node (i.e. typically once on mac, once per
install on windows) in addition to writing the root_dir.cap retrieved from
the native_client backend into a config file, it additionally writes a hash
thereof into the 'convergence' config file.

this causes uploads from this node to use a consistent 'convergence' hashing
value matching any other nodes with the same configured root_dir, i.e. for
the most part other systems installed and configured on the same account.