Refactor: fencer: Simplify schedule_stonith_command()

And add 1 to the modulus so that we include the full range of
(delay_max - delay_base), not that it really matters. For example, if
delay_max == 5 and delay_base == 2, then we want to add values in
{0, 1, 2, 3} to delay_base -- not to add values only in {0, 1, 2}.

Signed-off-by: Reid Wahl <nrwahl@protonmail.com>