engine : command infrastructure should know when the "execute" phase finished

When the engine is started after crashing/stopped while
a command with async tasks is running, we should know
after the restart whether the "Execution" phase of the
command has ended or not, so that we'll know whether
the command can be recovered or not.
Change-Id: Ie36e7d8a0263d5dd90fabe914a76711d5c001e72
Bug-Url: https://bugzilla.redhat.com/1118249
Signed-off-by: Ravi Nori <rnori@redhat.com>