Update self._stored.is_started where required

OSBaseCharm() registers an "update-status" event callback, but does not
update its own self._stored.is_started flag. By default this flag is
False and when the event gets delivered, the callback sends the charm
into a WaitingStatus().

This change adds "is_started = True" in our CinderCharmBase().

The places where the flag is set to True are up for discussion.
Alternatively, the base update_status() can be overridden, setting the
flag there before calling super().update_status()

Change-Id: I65c4b7477163617bb700889d300b10f66fcba152
diff --git a/src/charm.py b/src/charm.py
index 98a5225..2e935f8 100755
--- a/src/charm.py
+++ b/src/charm.py
@@ -85,6 +85,8 @@
         if conf_error is not None:
             logger.error(conf_error)
             self.unit.status = BlockedStatus(conf_error)
+            self._stored.is_started = False
+
             return
 
         self.create_storpool_conf(
@@ -95,10 +97,14 @@
 
         super().on_config(event)
 
+        self._stored.is_started = True
+
     def cinder_configuration(self, config):
         conf_error = self._check_for_config_errors(config)
         if conf_error is not None:
             logger.error(conf_error)
+            self._stored.is_started = False
+
             return []
 
         # Return the configuration to be set by the principal.
@@ -128,6 +134,8 @@
             )
         )
 
+        self._stored.is_started = True
+
         return options
 
     @staticmethod