upgrading percona cluster from ~15.04 to 17.11 fails while mistakenly attempting to rebootstrap the cluster

Upgrading from percona-cluster ~15.04 (r52, revision-id: <email address hidden>) to 17.11 fails as follows:

2018-02-01 03:26:12 INFO upgrade-charm * Stopping MySQL (Percona XtraDB Cluster) mysqld
2018-02-01 03:26:12 INFO upgrade-charm ...done.
2018-02-01 03:26:12 INFO upgrade-charm * Bootstrapping Percona XtraDB Cluster database server mysqld
2018-02-01 03:28:02 INFO upgrade-charm * The server quit without updating PID file (/var/run/mysqld/mysqld.pid).
2018-02-01 03:28:02 INFO upgrade-charm ...fail!
2018-02-01 03:28:02 INFO upgrade-charm Traceback (most recent call last):
[...]
2018-02-01 03:28:02 INFO upgrade-charm File "/var/lib/juju/agents/unit-mysql-0/charm/hooks/upgrade-charm", line 221, in render_config_restart_on_changed
2018-02-01 03:28:02 INFO upgrade-charm bootstrap_pxc()
[...]
2018-02-01 03:28:02 INFO upgrade-charm File "/usr/lib/python2.7/subprocess.py", line 1327, in _execute_child
2018-02-01 03:28:02 INFO upgrade-charm raise child_exception
2018-02-01 03:28:02 INFO upgrade-charm OSError: [Errno 2] No such file or directory

From reading the code and comparing two deployments, it seems that this is due to is_bootstrapped() mistakenly returning False because bootstrap-uuid is not set on the cluster relation:

def is_bootstrapped():
    [...]
        for unit in units:
            id = relation_get('bootstrap-uuid', unit=unit, rid=rid)
            if id:
                uuids.append(id)
    [...]
    if uuids:
        [...]
        return True
    return False

$ juju run --unit percona-cluster/0 'relation-get -r cluster:4 bootstrap-uuid percona-cluster/0'
$ juju run --unit percona-cluster-seventeen-eleven/0 'relation-get -r cluster:16 bootstrap-uuid percona-cluster-seventeen-eleven/0 '
aabb5dad-0702-11e8-9b4d-ee83bd330150
$ _