Add a config file option for setting the phased update

system-image-cli has a -p/--percentage option to explicitly override the device's phase percentage. There's no equivalent way to do this using system-image-dbus, but for testing purposes, it would be useful to be able to do this.

The only knobs to control the phase are the channel, the target revision, and the contents of a machine-id file (currently, /var/lib/dbus/machine-id and /etc/machine-id in that order, first come, first served). But this are awkward to use because they only serve as input to a random number generator, so can't be used to predict what the phase will end up being.

I think a good way to do this would be to add an optional [system]phase_percentage config file setting. This would have to be an integer between 0 and 100 and it would only override the system's calculated phase if given (it would not be set by default, but it would be documented in the ini manpage). It would work identically to s-i-cli's -p/--percentage switch.