Low: scheduler: Use default when series max value is out of bounds

Previously, the scheduler used crm_parse_ll() to parse the pe-*-series-max
cluster options, which meant that out-of-bounds values would be handled
unpredictably. (Invalid values correctly used the default because the value was
validated before calling crm_parse_ll().)

Now, use pcmk__scan_min_int(), so proper bounds checking is done.