leader-elected fails if leadership was lost in the meantime, breaking sync

[Paul Collins]

If a unit is deposed with the leadership-elected hook queued, and leadership-elected subsequently runs, it fails messily and blocks syncs, e.g.:

2017-09-07 20:52:43 ERROR juju.worker.dependency engine.go:547 "api-caller" manifold worker returned unexpected error: cannot open api: try again (try again)
2017-09-07 20:53:01 WARNING juju.worker.uniter.operation leader.go:115 we should run a leader-deposed hook here, but we can't yet
2017-09-07 20:53:02 INFO leader-elected error: cannot write leadership settings: cannot write settings: not the leader
2017-09-07 20:53:02 INFO leader-elected Traceback (most recent call last):
2017-09-07 20:53:02 INFO leader-elected File "/var/lib/juju/agents/unit-ubuntu-repository-cache-0/charm/hooks/leader-elected", line 245, in <module>
2017-09-07 20:53:02 INFO leader-elected HOOKS.execute(sys.argv)
2017-09-07 20:53:02 INFO leader-elected File "/var/lib/juju/agents/unit-ubuntu-repository-cache-0/charm/lib/charmhelpers/core/hookenv.py", line 715, in execute
2017-09-07 20:53:02 INFO leader-elected self._hooks[hook_name]()
2017-09-07 20:53:02 INFO leader-elected File "/var/lib/juju/agents/unit-ubuntu-repository-cache-0/charm/hooks/leader-elected", line 226, in leader_elected
2017-09-07 20:53:02 INFO leader-elected hookenv.leader_set(leader_id=hookenv.local_unit())
2017-09-07 20:53:02 INFO leader-elected File "/var/lib/juju/agents/unit-ubuntu-repository-cache-0/charm/lib/charmhelpers/core/hookenv.py", line 837, in inner_translate_exc2
2017-09-07 20:53:02 INFO leader-elected return f(*args, **kwargs)
2017-09-07 20:53:02 INFO leader-elected File "/var/lib/juju/agents/unit-ubuntu-repository-cache-0/charm/lib/charmhelpers/core/hookenv.py", line 890, in leader_set
2017-09-07 20:53:02 INFO leader-elected subprocess.check_call(cmd)
2017-09-07 20:53:02 INFO leader-elected File "/usr/lib/python3.5/subprocess.py", line 581, in check_call
2017-09-07 20:53:02 INFO leader-elected raise CalledProcessError(retcode, cmd)
2017-09-07 20:53:02 INFO leader-elected subprocess.CalledProcessError: Command '['leader-set', 'leader_id=ubuntu-repository-cache/0']' returned non-zero exit status 1
2017-09-07 20:53:02 ERROR juju.worker.uniter.operation runhook.go:107 hook "leader-elected" failed: exit status 1

This can be difficult to notice because by default Juju 2 will regularly retry the hook. Running "juju resolve --no-retry ubuntu-repository-cache/0" skips the hook and lets the sync complete properly.