Fix KeyError when 'current' is missing

[Cynerva]

Proposed fix for an error I've seen occasionally in our test runs:

Traceback (most recent call last):
  File "/tmp/bundletester-tMSZKP/tmpA7PRkx/tests/20-charm-validation.py", line 171, in test_etcd_scale_on_master
    self.deployment.sentry.wait()
  File "/usr/lib/python3/dist-packages/amulet/sentry.py", line 547, in wait
    if check_status(status):
  File "/usr/lib/python3/dist-packages/amulet/sentry.py", line 531, in check_status
    if unit['agent-status']['current'] != 'idle':
KeyError: 'current'

It looks like this can occur when calling sentry.wait() immediately after removing a unit. This is exactly what our failing test is doing: https://github.com/juju-solutions/bundle-canonical-kubernetes/blob/master/tests/20-charm-validation.py#L169-L171

The root cause seems to be that Juju, sometimes, does not include the current key within juju-status (which is normalized to agent-status in amulet). See this output that occurred during model destruction, where unit etcd/2 is missing the current key.

[tvansteenburgh]

LGTM. Good to merge once tests pass.