Implement generic share groups in Manila

Manila needs a grouping construct that, like shares, is a 1st-class atomic data type. Our experience with CGs has demonstrated the complexity of adding a grouping capability, yet there are other use cases such as migration, replication, and backup in which some storage controllers could only offer such features on share groups. CGs also highlighted the poor optics of an advanced feature with comparatively little potential for vendor support. And adding new grouping constructs for each new feature is not technically feasible. All of the above may be addressed by generic groups, which we think is a clean extension to the original architecture of Manila.