This change introduces and checks the COMPUTE_RESCUE_BFV trait that was
introduced in os-traits 2.2.0 in the compute layer during an instance
rescue when the instance boots from a volume.
An additional kwarg ``allow_bfv_rescue`` flag is also added to the
signature of the rescue method within the compute API. This defaults to
False and will be used in a following change to indicate when the
request is using a high enough microversion to invoke this new
capability.
The ``supports_bfv_rescue`` capability tracked within the virt drivers
that this trait maps to is only added to the powervm driver for now due
to the way in which these capabilities are checked by the
``TestPowerVMDriver.test_driver_capabilities`` test.
Implements: blueprint virt-bfv-instance-rescue
Change-Id: Ic2ad1468d31b7707b7f8f2b845a9cf47d9d076d5
5b6f44efff