simplify pool_instance, split instance schema

- simplify the communication from appliance to pool by using the
	PoolApiBase class in the pool_instance method
- get rid of all the not used stuff that i introduced for syncing
	flavors, which is all not used anymore now
- split the instance schema into instance and appliance. appliance
	schema will probably need to be reused in different context later
- introduce schema_mixin class which can be added to model classes to
	add an as_dict method to them. this method dumps their data as dict
	via the according schema