Rename 'slave' to 'replica'

Remove all references to the terms 'slave' and 'master, except in code
that is specific to MySQL. Use the terms 'replica' and 'replication
source' instead.

Add a db migration to rename the instances.slave_of_id column to
replica_of_id.

Update apischema.py to reflect support for replicas.

Depends-On: Id22d18a84a4ac104ff8af09959e8eb2ae0102e97
Related-Bug: 1360310
Change-Id: I68f6c5f2fdb210afcbad504df4cf0588400fc42a
Fix NoSuchOptError on Couchbase create

A previous commit moved the backup_incremental_strategy option from
DEFAULT group to datastore specific grouping. However, the option was
only implemented for Mysql and Percona so when any datastore that had
backup support (like Couchbase) attempted to start, it received a
NoSuchOptError because of the missing option.

This fix put an empty dict in for the option for all datastores that don't
currently have backup or incremental backup support implemented.

Note: this wasn't picked up by any automated tests because the gate currently
only tests Mysql.

Closes-Bug: 1366175