Add assess_status() to base layer as part of workload status

This bakes in the assess_status() to the update-status hook to call
assess_status() on the charms_openstack.charm.OpenStackCharm class.

This has the effect of determining the workload status for the
unit.

Change-Id: Idf9a712f818445ba9f68dccae2d98237eb7f7eee

reactive/charms_openstack_handlers.py

@@ -0,0 +1,22 @@
+# handle the update-status hook for all openstack charms
+
+from __future__ import absolute_import
+
+import charms.reactive as reactive
+import charmhelpers.core.hookenv as hookenv
+
+import charms_openstack.charm as charm
+
+
+@reactive.hook('update-status')
+def update_status():
+    """Use the update-status hook to run the assess_status() function for the
+    unit.
+
+    This runs, via the singleton, the assess_status() on the derived class,
+    which is auto-instantiated according the the release.
+
+    To deactivate this function override the assess_status in the derived class
+    to a NOP.
+    """
+    charm.OpenStackCharm.singleton.assess_status()