core:[ExternalTasks] When adding new step, type..

[ExternalTasks] When adding new step, type is always EXECUTING

Changing AddExternalStepCommand::execute to get the step type from the
parameters object instead of using a constant EXECUTING value

Change-Id: I04134ffa8e937e623aa7107ca501fb7e79a3ab5c
BUG-Url: https://bugzilla.redhat.com/show_bug.cgi?id=988086
Signed-off-by: Eli Mesika <emesika@redhat.com>