Allow to automatically reserve an FSM test id

Allow to automatically reserve an FSM test id

We will allow to provide '0' as special value for test
identifier on test manager in order to reserve the id
as the current pool size plus 1. This is valid if previous
reservations were done in the same way, or starting from
the id=1 and monotonically increased without holes in the
sequence.

REST API now will return the response body with the response field
as the affected test id in order to know the one reserved just in
case 0 was provided instead of specific value.

Component tests have been adapted to the new feature.