Extend options for custom flavor sorting when launching instances

Until now, flavors were sorted by RAM, ascending, by default.
An option to override this and use a custom callable instead
was introduced via the CREATE_INSTANCE_FLAVOR_SORT dict in
local_settings.py.

This patch introduces another way of using the
CREATE_INSTANCE_FLAVOR_SORT dict: by providing a flavor attribute.

Change-Id: I5b2819cc3ebc97377e673682d0b2c7ddab94e3d3
Closes-Bug: #1254524
Merge "Extend options for custom flavor sorting when launching instances"