Merge firstboot KVStore entries

Created by: fonfon

We had two kvstore entries for representing whether firstboot is completed: setup_state and the legacy firstboot_state. Both are booleans and the name is not accurate for what they are/were doing anymore.

• merge kvstore entries 'firstboot_state' and 'setup_state' into 'firstboot_completed' (a hopefully more accurate name). This is done via a migration so we don't have to deal with legacy fields in our code.
• make more use of functionality provided by firstboot.__init__.py instead of dealing with kvstore directly
• introduced kvstore.delete (needed by the migration script to clean old entries)