v2: acme-dns-tiny: if a CNAME is defined on _acme-challenge.example.org...

v2: acme-dns-tiny: if a CNAME is defined on _acme-challenge.example.org following it to decide where to install the TXT records

Limitation: it only follows one CNAME for the moment, maybe we should
go through CNAME chain, but it will requires to detect CNAME loop too.
3 jobs for v2 in 3 minutes and 28 seconds (queued for 1 minute and 17 seconds)
Status Job ID Name Coverage
  Test
failed #369
jessie

00:01:31

failed #370
jessie_backport

00:01:04

failed #371
stretch

00:00:53

 
Name Stage Failure
failed
jessie_backport Test There is an unknown failure, please try again
  File "/builds/adrien/acme-dns-tiny/tests/test_acme_dns_tiny.py", line 45, in assertCertificateChain
self.assertEqual(3, len(certlist))
AssertionError: 3 != 1

----------------------------------------------------------------------
Ran 12 tests in 24.907s

FAILED (failures=2, errors=10)
ERROR: Job failed: exit code 1
failed
jessie Test There is an unknown failure, please try again
  File "/builds/6ecef716/0/adrien/acme-dns-tiny/tests/test_acme_dns_tiny.py", line 45, in assertCertificateChain
self.assertEqual(3, len(certlist))
AssertionError: 3 != 1

----------------------------------------------------------------------
Ran 12 tests in 26.746s

FAILED (failures=2, errors=10)
ERROR: Job failed: exit code 1
failed
stretch Test There is an unknown failure, please try again
  File "/builds/adrien/acme-dns-tiny/tests/test_acme_dns_tiny.py", line 45, in assertCertificateChain
self.assertEqual(3, len(certlist))
AssertionError: 3 != 1

----------------------------------------------------------------------
Ran 12 tests in 24.920s

FAILED (failures=2, errors=10)
ERROR: Job failed: exit code 1