Fix issues with cells and resize

There are cases where compute API resizez() is called with 'flavor_id'
as a positional argument, so we need to fix up the cells_api version of
resize() to more closely match the super class.

Also: A recent change for storing instance_type info in system_metadata
happened to make an incorrect change, assuming cells_api was for child
cells.. which it's for the API cell.  It needs to pull the instance type
from the DB just like the super class does.

Fixes bug 1154843

Change-Id: Ifdf2a68c3bff6d27d5f192e2eb73f9b79804c2c7
Merge "Fix issues with cells and resize"