[nova][ptg]Support volume local cache

spec: https://review.opendev.org/#/c/689070/

TODOs:

LiangFang to update the spec based on the discussion in the room[1]: * use traits to driver scheduling. The cache is not sliced per instance so it cannot be a resource class * document the alternative between doing a hard scheduling decision or only implement caching as a best effort optimization for the guest. * document the alternative to do the whole cache management on libvirt (or QEMU) level

[1] https://etherpad.openstack.org/p/nova-shanghai-ptg