core: ExternalSchedulerDiscoveryResult map iter

Replaced keySet() iteration with entrySet() iteration to improve
performance.

This patch fixes the WMI_WRONG_MAP_ITERATOR FindBugs warning on this
class.

Change-Id: I27d1a45bbc184bf467a283f0932a27db8c880831
Signed-off-by: Allon Mureinik <amureini@redhat.com>