engine : Children of callback commands should not have callback enabled by default

Children of commands with callback should have callback
enabled only if they have custom callbacks implemented

Change-Id: Ib6c0f1e42572dfa51b50d2e6cc9531a81c2f5aad
Bug-Url: https://bugzilla.redhat.com/1133041
Signed-off-by: Ravi Nori <rnori@redhat.com>