Improve use of systemd's states

A better handling of systemd state is need to treat
correctly transient states. That change includes:

- Management of states with numeric identifiers
  instead of names

- Handling of the state "inactive" as a stable
  state. Most of previous seen problems were coming
  from that miss.

- Returning no error but also no info on the process
  if it falled to "inactive" meaning that it stopped
  quickly.

Bug-AGL: SPEC-3323

Change-Id: Ibf35eb6257c5583596d675cad0bec2869f5fd5f7
Signed-off-by: José Bollo <jose.bollo@iot.bzh>