Varnish: set hash_ignore_busy for CentralAutoLogin

Despite the English version of CentralAutoLogin being randomized across
backends, SpecialAutoLogin URLs accounted for about 23% of all 503s for
the period of Nov 26th-29th; 48% was the non-backend_random-yet German
version of CentralAutoLogin and the rest of the URLs accounted for <= 1%
each.

These URLs are, popular but their overpresentation is not explained by
access logs: for the same period account, Special:CentralAutoLogin
accounted only for 5.5% of sampled-1000s. These are the most popular
misses, though, which means that the increased failure rate is either
some underlying backend problem or increased contention on the Varnish
side.

Set hash_ignore_busy = true for English/German CentralAutoLogin and hope
that this will help with possible contention, especially in combination
with the backend_random fix.

Change-Id: I27acac463315a8b5628f65a56f1bb3be7aa10b8a