Add common base weigher/weigher handler for filter scheduler

Filter scheduler is being used for more than one core projects (Nova
and Cinder as of writing), the implementation shared a lot of common
code. This patch is to move base weigher (weighing function), weigher
handler for filter scheduler into oslo to reduce possible porting.

implement bp: common-weights

Change-Id: I2d1b37438663b53e035cc262875a283e5e2ee970
Merge "Add common base weigher/weigher handler for filter scheduler"