core: CommandBase modifications

Adding handling of external step & sub-step
Fixes NPE in CommandBase and adding external task handling to
ExecutionHndler.

Change-Id: Ic6350853f66e8f117dc3ebf8ff3e346bb9b241ad
Signed-off-by: Eli Mesika <emesika@redhat.com>
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=872719