Support for phased updates

The relevant function in update-manager appears to be:
    def _is_ignored_phased_update(self, pkg):
        """ This will test if the pkg is a phased update and if
            it needs to get installed or ignored.

            :return: True if the updates should be ignored
        """
        # allow the admin to override this
        if apt.apt_pkg.config.find_b(
                self.ALWAYS_INCLUDE_PHASED_UPDATES, False):
            return False

        if self.PHASED_UPDATES_KEY in pkg.candidate.record:
            if apt.apt_pkg.config.find_b(
                    self.NEVER_INCLUDE_PHASED_UPDATES, False):
                logging.info("holding back phased update per configuration")
                return True

            # its important that we always get the same result on
            # multiple runs of the update-manager, so we need to
            # feed a seed that is a combination of the pkg/ver/machine
            self.random.seed("%s-%s-%s" % (
                pkg.candidate.source_name, pkg.candidate.version,
                self.machine_uniq_id))
            threshold = pkg.candidate.record[self.PHASED_UPDATES_KEY]
            percentage = self.random.randint(0, 100)
            if percentage > int(threshold):
                logging.info("holding back phased update (%s < %s)" % (
                    threshold, percentage))
                return True
        return False

The unique id comes from:
        UNIQ_MACHINE_ID_FILE = "/var/lib/dbus/machine-id"
        with open(self.UNIQ_MACHINE_ID_FILE) as f:
            self.machine_uniq_id = f.read()

Note that we have that file on Ubuntu Touch too and it's writable, persistent and appaears to be indeed unique, so I'd recommend we just use that.

Implementation notes:

* The calculated phased percentage will be the same for the entire life of the system-image-{cli,dbus} process. I.e. it won't be different for each image comparison we see in the index.json file.

* We'll read /var/lib/dbus/machine-id as bytes (including the trailing newline) as the random seed. This means it would be the same for every invocation of system-image. If we want it to be different each time (and we indeed probably do), then we need to hash the contents of that file, and add in something like the current system time in float seconds.

E.g.:

import time
import random

with open('/var/lib/dbus/machine-id', 'rb') as fp:
    data = fp.read()

now = str(time.time()).encode('us-ascii')
seed = data + now

r = random.Random()
r.seed(seed)
for i in range(10):
    print(r.randint(0, 100))