Re-use GRAPHITE_TIME_FORMAT constant

[lunkwill42]

Look into re-using the GRAPHITE_TIME_FORMAT constant introduced into nav.web.sortedstats by #2398, where applicable, such as:

nav/python/nav/metrics/data.py

Lines 108 to 111 in 78e03b2

	if isinstance(start, datetime):
	start = start.strftime('%H:%M%Y%m%d')
	if isinstance(end, datetime):
	end = end.strftime('%H:%M%Y%m%d')

Originally posted by @lunkwill42 in #2398 (comment)