Global Host Reservations Task 5: data migration scripts to convert existing subnet-id values of 0

The changes made in 5704 to support global HRs necessitate migrating existing data. Specifically for MySQL and PostgreSQL, any columns with values of 0 for subnet IDs in hosts and options tables, need to replace with NULL, and for Cassandra, they should be replaced with GLOBAL_ID_UNUSED.

Data migration steps need to be added to the schema upgrade scripts for 1.5.0 to accommodate this.

Replaces http://kea.isc.org/ticket/5708

Edited Aug 15, 2018 by Thomas Markwalder

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