Move from generator to iterator for iterjobs

Instead of yielding back valid jobs, alter the function
to return a iterator which can support various capabilities
when iterated over. These capabilities can be changed during
iteration and are more flexible in allowing new jobboard
implementations to change there supported iteration
capabilities.

Change-Id: Ibcd47d881a5c8689b44bc444402f51030649c0be
Merge "Move from generator to iterator for iterjobs"