Remove indexes that are prefix subsets of other indexes

Both MySQL and PostgreSQL can efficiently optimize queries using
indexes that use the prefix columns of the index. This means that
an index that covers columns (a, b, c) can be used to optimize
a query with the statement 'WHERE a = 1 and b = 9'.

Nova has indexes which are a prefix subset of other indexes. The
database servers can use the suffix superset indexes and the prefix
subset indexes just add extra overhead to inserts and updates.

instances_host_deleted_idx (host, deleted) is a prefix subset of
instances_host_deleted_cleaned_idx (host, deleted, cleaned)

iscsi_targets_host_idx (host) is a prefix subset of
iscsi_targets_host_volume_id_deleted_idx (host, volume_id, deleted)

Change-Id: I397c7f1ba9ab95a99f13c9ec6e838ae63987870b
Merge "Remove indexes that are prefix subsets of other indexes"