Add common filter/filter 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 moves base filter/filter handler class as well as
common filter implementation for filter scheduler into oslo to reduce
possible porting.

implement bp: common-filters

Change-Id: If0b1dee79c410c98e152230b55c1ec5dbcdef27c
Merge "Add common filter/filter handler for filter scheduler"