Implemente way to send diagnostic request when subscribed.

Implemente way to send diagnostic request when subscribed.

When subscribed, the signal is added to recurring request list
of diagnostic manager and an event is added to the systemd
event loop with timer set using frequency parameter from the
requested signal.

Change-Id: I4d604c498047d7744c090b7f03fce0f2b427fd01
Signed-off-by: Romain Forlot <romain.forlot@iot.bzh>