[PATCH 6/9] powercapping : correct a bug in overlapping reservation management

The previous power reservation check, replicated from job_test_lic_resv
is not well protected against consecutives reservations for which
reserved watts (or licenses count) can not be added directly.
Thus, if the job overlaps multiple non-overlapping reservations,
it will be prevented to use more watts (or licenses) than necessary.

Correct this bug building a planning of the evolution of the
constraints and then extracting the max amount of watts reserved
by potentially overlapping reservations.

Also correct a bug resulting in valid power check when more watts
than the defined powercap were reserved.

(Adapted for 15.08 by Yiannis Georgiou <yiannis.georgiou@bull.netwq>)