Composable flavor properties

Flavor, the instance hardware template, record ``ram/cpu/disk`` related
information.
Currently, it takes on too much responsibility for instance-related metadata
(key, value), even if many metadata information not related to the flavor is
placed in `extra spec`_, due to these extra spec exist, the flavor looks too
bloated and highly coupled.

If some (key, value) option is configured in the ``extra_spec`` of the flavor,
that flavor will be used as a dedicated flavor. For example, a flavor is named
'foo-flavor'. When the memory large page configuration item
``{"hw:mem_page_size": "1GB"}`` is set in 'foo-flavor', select 'foo-flavor'
when creating the instance. The instance will can only be booted on the node
where the large page of memory is configured, otherwise it will fail to start.

I think it's a problem that the simple details of a flavor (cpu, memory, disk)
are correlated to the optional complex details, such as numa or hugepages.