common/factory: add function to get registered keys
common/factory: add function to get registered keys

Add a keys() function to get the registered keys from
the factory.

Signed-off-by: U. Artie Eoff <32aece0d55055227b44d04b7d0051cf3d04ee871@intel.com>

seanvk/libyami,YuJiankang/libyami,zhaobob/libyami,chivakker/libyami,yizhouwei/libyami,lizhong1008/libyami,YuJiankang/libyami,zhaobob/libyami,stripes416/libyami,01org/libyami,halleyzhao/libyami,lizhong1008/libyami,seanvk/libyami,01org/libyami,seanvk/libyami,dspmeng/libyami,uartie/libyami,clearlylin/libyami,halleyzhao/libyami,zhaobob/libyami,stripes416/libyami,andyLiuChina/libyami,dspmeng/libyami,halleyzhao/libyami,andyLiuChina/libyami,uartie/libyami,andyLiuChina/libyami,chivakker/libyami,clearlylin/libyami,lizhong1008/libyami,dspmeng/libyami,yizhouwei/libyami,halleyzhao/libyami,chivakker/libyami,YuJiankang/libyami,clearlylin/libyami,01org/libyami,yizhouwei/libyami,stripes416/libyami,andyLiuChina/libyami,uartie/libyami