Capacity check during snapshot creation

Snapshot creation today does not evaluate the free space available at the storage backend. Due to this an uncontrolled snapshot creation can fill up the backend unwillingly and can lead disruptive effects at storage.
This happens because the call from API is directly sent to the driver and the driver with no checks issues the backend snapshot creation.
There are 2 ways to solve this:
1. Fix the snapshot creation code in each driver to check for the available space OR
2. Make this API to go through scheduler which will validate the available space and then pass the request to the driver.

It wont be easy to control the drivers and make changes to each of the driver, hence, via this bp I am proposing to change the API flow where cinder snapshot-create will now call the scheduler and validate capacity. If, enough exists then pass the call to driver else error out at the scheduler.