[nova][sfe] Support volume local cache

Hi We would like to have a spec freeze exception for the spec: Support volume local cache [1]. This is part of cross project contribution, with another spec in cinder [2]. I will attend the Nova meeting on February 20 2020 1400 UTC. [1] https://review.opendev.org/#/c/689070/ [2] https://review.opendev.org/#/c/684556/ Regards Liang