core: minor refactor for custom props validation

instead of calling one method to check the properties and then another
to check the result, now calling one method to do both.

Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1166632
Change-Id: I444f374d0c1bb4c495b3c2b1c26c92bc466e19c9
Signed-off-by: Omer Frenkel <ofrenkel@redhat.com>