Add possibility to boot service VMs from volume using generic driver

Nova allows to boot VMs in several possible ways. Manila generic share driver uses only one of them - boot from image.
It will be very useful in some cases to be able to boot VMs from volumes. One of such cases is when we want to have all data of VM to keep in Cinder, where we can backup it, restore and migrate.

High-level change is following:
- generic driver gets request for share-server creation
- generic driver sends request to cinder to create volume from image that is specified in CONFig as service image
- generic driver uses created volume to boot VM from

Above logic should be optional. So, some new config option will be available that will define whether we create VM from service image or volume based on this image.