blueprint host-aggregates

This is the first of a series of commits that add the host-aggregates capability,
as described on the blueprint page.

This commit, more precisely, introduces changes to the Nova model: model classes
related to aggregates have been added, as well as DB API methods to interact with
the model; a sqlalchemy migration script plus a bunch of tests are also part of
this changeset.

Commits that will follow are going to add:

- Extensions to OSAPI Admin, and related python_novaclient mappings
- Implementation of the XenAPI virt layer
- Integration of OSAPI and virt layer, via the compute_api
- smoketests
- openstack-manuals documentation

These commits will be pushed for review not necessarily in this exact order.

Change-Id: Iceb27609dc53bf4305c02d7cbc436fba4c4a7256

nova/exception.py

@@ -884,3 +884,28 @@ class WillNotSchedule(NovaException):
 class QuotaError(ApiError):
     """Quota Exceeded."""
     pass
+
+
+class AggregateNotFound(NotFound):
+    message = _("Aggregate %(aggregate_id)s could not be found.")
+
+
+class AggregateNameExists(Duplicate):
+    message = _("Aggregate %(aggregate_name)s already exists.")
+
+
+class AggregateHostNotFound(NotFound):
+    message = _("Aggregate %(aggregate_id)s has no host %(host)s.")
+
+
+class AggregateMetadataNotFound(NotFound):
+    message = _("Aggregate %(aggregate_id)s has no metadata with "
+                "key %(metadata_key)s.")
+
+
+class AggregateHostConflict(Duplicate):
+    message = _("Host %(host)s already member of another aggregate.")
+
+
+class AggregateHostExists(Duplicate):
+    message = _("Aggregate %(aggregate_id)s already has host %(host)s.")