Allow non-public Postgres schema

[zhukau]

If using custom schema in Postgres, i.e. - kong, instead of public Kong works until first restart/reload.
Then it fails with error message Error during migration 2015-01-12-175310_init_schema: ERROR: relation "custom_id_idx" already exists caused by hardcoded public schema name in database objects existence checks.

While it is nice to have a configurable schema name as suggested in issue #1193, in reality we can rely on search_path configured on database level and its default value $user, public that covers 99% of use cases.