Add Host Mapping table to API Database

This patch creates the host_mappings table in the API database for cellsv2.
This table will contain mapping of compute hosts to cells.

Change-Id: I1bd35434ce186e1fb49d56c621bff3f09aa90138
Implements: blueprint cells-host-mapping

nova/db/sqlalchemy/api_migrations/migrate_repo/versions/003_host_mapping.py

@@ -0,0 +1,43 @@
+#    Licensed under the Apache License, Version 2.0 (the "License"); you may
+#    not use this file except in compliance with the License. You may obtain
+#    a copy of the License at
+#
+#         http://www.apache.org/licenses/LICENSE-2.0
+#
+#    Unless required by applicable law or agreed to in writing, software
+#    distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+#    WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+#    License for the specific language governing permissions and limitations
+#    under the License.
+
+from migrate.changeset.constraint import ForeignKeyConstraint
+from migrate import UniqueConstraint
+from sqlalchemy import Column
+from sqlalchemy import DateTime
+from sqlalchemy import Index
+from sqlalchemy import Integer
+from sqlalchemy import MetaData
+from sqlalchemy import String
+from sqlalchemy import Table
+
+
+def upgrade(migrate_engine):
+    meta = MetaData()
+    meta.bind = migrate_engine
+
+    cell_mappings = Table('cell_mappings', meta, autoload=True)
+    host_mappings = Table('host_mappings', meta,
+        Column('created_at', DateTime),
+        Column('updated_at', DateTime),
+        Column('id', Integer, primary_key=True, nullable=False),
+        Column('cell_id', Integer, nullable=False),
+        Column('host', String(length=255), nullable=False),
+        UniqueConstraint('host',
+            name='uniq_host_mappings0host'),
+        Index('host_idx', 'host'),
+        ForeignKeyConstraint(columns=['cell_id'],
+            refcolumns=[cell_mappings.c.id]),
+        mysql_engine='InnoDB',
+        mysql_charset='utf8'
+    )
+    host_mappings.create(checkfirst=True)

nova/db/sqlalchemy/api_models.py

@@ -56,3 +56,16 @@ class InstanceMapping(API_BASE):
     cell_id = Column(Integer, ForeignKey('cell_mappings.id'),
             nullable=False)
     project_id = Column(String(255), nullable=False)
+
+
+class HostMapping(API_BASE):
+    """Contains mapping of a compute host to which cell it is in"""
+    __tablename__ = "host_mappings"
+    __table_args__ = (Index('host_idx', 'host'),
+                      schema.UniqueConstraint('host',
+                        name='uniq_host_mappings0host'))
+
+    id = Column(Integer, primary_key=True)
+    cell_id = Column(Integer, ForeignKey('cell_mappings.id'),
+            nullable=False)
+    host = Column(String(255), nullable=False)