amulet doesn't provide a clean way to refresh unit info

If you reconfigure a service with e.g. a port change,
amulet doesn't reload/refresh unit_data, keeping its sentry
entries stale.

Example excerpts (charm code unchanged between both):

* FAIL:
self.deployment.configure('myservice', {'port': '12345'})

self.unit = self.deployment.sentry['myservice/0']
ports = self.unit.info['open-ports']
## below FAILs, as ports hold default values as already deployed
self.assertEqual(ports, ['12345/tcp'])

* OK (workaround'd):
self.deployment.configure('myservice', {'port': '12345'})

## Refresh units_data by re-creating sentry object
from amulet.sentry import Talisman
self.deployment.sentry.wait()
self.deployment.sentry = Talisman(self.deployment.services)

self.unit = self.deployment.sentry['myservice/0']
ports = self.unit.info['open-ports']
## Ok now:
self.assertEqual(ports, ['12345/tcp'])

Suggest making wait() to re-create sentry object (as
e.g. add-unit() does), or exposing a new method to do so,
e.g. deployment.refresh().