Migrate probes to use the Rucio `gateway` layer instead of `core`

[rdimaio]

e.g. this probe:

probes/common/check_free_space

Lines 18 to 36 in a7c0556

	from rucio.core.rse import list_rses, get_rse_limits, set_rse_usage
	# Exit statuses
	OK, WARNING, CRITICAL, UNKNOWN = 0, 1, 2, 3
	if __name__ == "__main__":
	try:
	for rse in list_rses():
	limits = get_rse_limits(rse=rse['rse'], rse_id=rse['id'])
	min_free_space = limits.get('MinFreeSpace')
	if min_free_space is not None:
	# print rse['rse'], min_free_space
	set_rse_usage(rse=rse['rse'], source='min_free_space', used=min_free_space, free=None)
	except:
	print (traceback.format_exc())
	sys.exit(UNKNOWN)
	sys.exit(OK)

Should use rucio.gateway methods, rather than rucio.core. See rucio/rucio#6717 (comment) for reference