Add a mechanism to execute external command from a workflow

[safchain]

This could be done like for the sleep function https://github.com/skydive-project/skydive/blob/master/api/server/workflow_runtime.go#L62-L68

[lebauce]

One possibility would be to export a execute function in the same way than the sleep function : https://github.com/skydive-project/skydive/blob/master/api/server/workflow_runtime.go#L62-L68

[lebauce]

cc @andrekassis

[eranra]

@safchain @lebauce thanks ... we will share this with the students ...I agree this looks much less complicated than I originally thought,.