Remove _instance_update usage in _default_block_device_names

We have an instance object in _default_block_device_names so
we can just call instance.save() directly.  We don't need to
update resource tracker stats so there is no need to call
_instance_update in this method.

Part of blueprint compute-manager-objects-juno

Change-Id: Ia8faa67a36ff31a3ba526770f5d8a0361326ffc7
Treat instance like an object in _build_instance

The _build_instance method has an instance object already
but treats it like a primitive dict.  This change uses
dot notation to access fields on the object and updates
the unit tests to use instance objects.

Part of blueprint compute-manager-objects-juno

Change-Id: I19c5abc502b57579b4706e8a8e7b7a64f6f9b6ca
Remove _instance_update usage in _build_instance

The _build_instance method has an instance object so
just call instance.save() rather than _instance_update
which does a separate call to conductor to update the
instance updates the resource tracker, which is
unnecessary in this case. Compare to the _build_resources
method.

Part of blueprint compute-manager-objects-juno

Change-Id: Ic350a424f83638e8129d55dda168913798ebc31a
Merge "Remove _instance_update usage in _default_block_device_names"
Merge "Treat instance like an object in _build_instance"
Merge "Remove _instance_update usage in _build_instance"