Description of problem:
Allow to change shutdown=remove to shutdown=preserve on already existing instances as some users might change their mind after creating an instance that would delete the created volumes (default behavior) with:

   $ openstack server create --flavor 2 --volume VOLUME_ID \
  --block-device source=volume,id=$VOLUME_ID,dest=volume,size=10,shutdown=remove,bootindex=0 \
  myInstanceFromVolume

In the user decides he wants to keep the volume after deleting the VM, there're no ways of altering this right but to temper with the database manually.  Would it be possible to add something like "nova update" and allow user to change that value?

Version-Release number of selected component (if applicable):


How reproducible:
Always

Steps to Reproduce:
1. Create a boot from volume VM
2. Delete the VM

Actual results:
Boot volume is deleted

Expected results:
Keep it

Additional info:
Users sometime changes their mind.