Client removes config values from config files if they match defaults.

[Geoff Teale]

This bug splits out one aspect of bug #1216696 that can be easily fixed: currently whenever the landscape client modifies its config file it writes a new file, and omits any config values that match the defaults and weren't explicitly passed on the command line. To fix this issue we simply need to respect values that already existed in the config file in the same way we respect options passed on the command line (whilst still giving the command line precedence). That intended functionality will therefore be that we don't write *additional* default values to the file.