Check if volume is bootable when creating an instance

Cinder provides a bootable flags for volumes that's not being
checked when creating an instance.

If a user select a non-bootable volume, then the VM became unusable
since nothing will be booted at all.

This check is done only in case that a volume is selected as root
device for a VM.  Boot from snapshot will still work (even that the
volume will be marked as non-bootable by cinder, but will boot the
attached snapshot instead)
If the attribute bootable is not present, we keep the old behaviour
and allow the instance creation.

Change-Id: I8e028d40d6a11beeac4f043afccb17eb61f6ce62
Closes-Bug: #1321334
Merge "Check if volume is bootable when creating an instance"