Do not have periodic tests default domains

[bwbroersma]

• Add periodic test to monitor for operational problems  #1245 accidentally created some new issues

The test run if cron is in the COMPOSE_PROFILE and CRON_15MIN_RUN_TESTS is True, which is a default and disabled in develop.env:

Internet.nl/docker/defaults.env

Lines 261 to 262 in 2b50952

	# enable running tests every 15 minutes for metrics collection
	CRON_15MIN_RUN_TESTS=True

With these default non configurable domains:

Internet.nl/docker/cron/periodic/15min/tests.py

Lines 38 to 63 in 2b50952

	TEST_DOMAINS = {
	# domain's to use in website tests
	"site": [
	"internet.nl",
	"example.nl",
	"example.com",
	"internetsociety.org",
	"ripe.net",
	"surf.nl",
	"ecp.nl",
	"forumstandaardisatie.nl",
	"minez.nl",
	],
	# domain's to use in mail tests
	"mail": [
	"internetsociety.org",
	"ripe.net",
	"surf.nl",
	"ecp.nl",
	# these are currently really slow and will probably improve when
	# we switch to sslyze, for now disable these in monitoring
	# "internet.nl",
	# "forumstandaardisatie.nl",
	# "minez.nl",
	],
	}

This means every production instance (single test, development single test, batch, development batch) will periodically test these domains, next to all production instances not managed by Platform Internetstandards.

The domains should be configurable and not have a set of default domains.
Next to this. the domains used for performance issues used by Platform Internetstandards should be informed.