Adds option to rebuild instance with existing disk

Evacuation of VM from a failed host for HA.
New api supports recreate/rebuild modes. With shared storage,
VM is recreated on the specified target from existing disk.
Otherwise, rebuild from original image is performed.

* new recreate, target host and on_shared_storage in rpcapi.rebuild
* change in manager.rebuild to rebuild instance using existing disk

part of blueprint rebuild-for-ha

DocImpact

Change-Id: I4d119eafc30da2779ad252259f88118ed0038cd1
Co-authored-by: Oshrit Feder <oshritf@il.ibm.com>
Fixed typos in doc strings.

Change-Id: Ibf4bf4ce882077c89d8bb91f93572af7b6cc5da2
Merge "Fixed typos in doc strings."
Merge "Adds option to rebuild instance with existing disk"