Add mysql_engine and mysql_charset options to shared_zones table

Shared_zones tables is the only table not defining the options
`mysql_engine` and `mysql_charset`, leading into an error if database
default charset is different from the one defined in the other tables.

Change-Id: I1d254553d40a50be264d67e1a8e81fa31f7a4c9f

designate/storage/sqlalchemy/tables.py

@@ -196,6 +196,9 @@ shared_zones = Table(
     UniqueConstraint('zone_id', 'project_id', 'target_project_id',
                      name='unique_shared_zone'),
     ForeignKeyConstraint(('zone_id',), ['zones.id'], ondelete='CASCADE'),
+
+    mysql_engine='InnoDB',
+    mysql_charset='utf8'
 )
 
 recordsets = Table('recordsets', metadata,