API to query reservation candidate

Blazar users needs to know the candidates of time series for their reservations before they create their reservation. With current lease create API, an user only knows the acceptance of their reservation by the API response. If the request fails, they need to change reservation's properties, start_time or end_time and call the API again until the request is accepted.

The reservation candidate API returns a list of candidate to be able to be reserved by the user. The users can query the candidate of reservation before they create their lease/reservation. It will reduce efforts in lease create phase.

See the details in https://etherpad.openstack.org/p/blazar-ptg-rocky