initial commit, fleshed out basic interface function, readme, etc

README.md

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+# Overview
+
+Basic interface for sending Cinder subordinate backend configuration to
+principle Cinder charms.
+
+# Usage
+
+## Requires
+
+This interface layer will set the following state:
+
+  * `{relation_name}.connected` The relation is established, but the charm may
+    not have provided any backend information.
+
+For example, the subordinate would handle the `cinder-backend.connected` state
+with something like:
+
+```python
+@when('cinder-backend.connected')
+def configure_cinder(cinder_principal):
+    config = {'api-endpoint': '1.2.3.4',
+              'admin-username': 'admin',
+              'admin-password': 'openstack',
+              'api-version': '1.0'}
+    cinder_principle.configure_principal(
+      backend_name='my_backend', configuration=config)
+```
+
+
+# Contact Information
+
+- <michael.skalka@canonical.com>

interface.yaml

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+name: cinder-backend
+summary: OpenStack Cinder backend interface
+version: 1
+repo: https://github.com/mskalka/interface-cinder-backend

provides.py

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+#!/usr/bin/python
+
+import json
+
+from charms.reactive import RelationBase
+from charms.reactive import hook
+from charms.reactive import scopes
+
+
+class CinderBackendProvides(RelationBase):
+
+    scope = scopes.GLOBAL
+
+    @hook('{provides:cinder-backend}-relation-joined')
+    def cinder_backend_joined(self):
+        conv = self.conversation()
+        conv.set_state('{relation_name}.joined')
+        self.set_state('{relation_name}.connected')
+        self.set_state('{relation_name}.available')
+
+    @hook('{provides:cinder-backend}-relation-{broken, departed}')
+    def cinder_backend_departed(self):
+        conv = self.conversation()
+        conv.remove_state('{relation_name}.joined')
+        self.remove_state('{relation_name}.available')
+        self.remove_state('{relation_name}.connected')
+        conv.set_state('{relation_name}.departing')
+
+    def configure_principal(self, backend_name, configuration):
+        """Send principle cinder-backend information"""
+        conv = self.conversation()
+
+        subordinate_configuration = {
+            "cinder": {
+                "/etc/cinder/cinder.conf": {
+                    "sections": {
+                        backend_name: configuration
+                    }
+                }
+            }
+        }
+
+        conv.set_remote(backend_name=backend_name
+                        subordinate_configuration=configuration)