Fix enums usage for postgres in migrations

For downgrade with postgres it is not enough to just drop_table if
table contains enums. The enums should be removed separately after
deleting the table.

Also, if a migration uses an existing enum then postgresql needs some
specific code to re-use this enum.

Closes-Bug: #1243213

Change-Id: Ie5bb8282f7b289c917679192a0d722eb29b8d30f
Merge "Fix enums usage for postgres in migrations"