Extend snapd REST API to get conf props from change transaction

[Vladimir Kiryakov]

Expose api to get just setted props from `change transaction` storage. Should be accessible in configure script to get just changed/setted props.

/v2/snaps/[name]/conf has two type op executions
- no options - returns all snap conf props from persistent storage.
- request for specific option - returns option from `change transaction` storage.
- Add new one to get from `change transaction` storage.

[Vladimir Kiryakov (vkiryakov)]

Extend API to use Changes function in `overlord/configstate/transaction.go `func (t *Transaction) Changes() []string {`.
It returns changing keys associated with this transaction.

We use in the same approach in unit testing.
overlord/configstate/transaction_test.go t.Changes()