Inconsistent database indexes exist in mysql and other databases

[Rui Chen]

A lot of database indexes are created for mysql in db migrating file 216_havana.py (see mysql_pre_indexes), and the part of indexes are added in 271_sqlite_postgresql_indexes.py in order to keep the consistence between mysql and other databases, but it still exists some different indexes between mysql and other databases, like: Index('instance_type_id', instance_type_projects.c.instance_type_id), Index('address', fixed_ips.c.address), Index('instance_uuid', instance_system_metadata.c.instance_uuid) and so on.

It cause an issue, the huge performance difference exists when we use different type databases (for example: mysql, postgresql).

At least, we should keep the consistent indexes in all supported databases.