Disable ipv6 in sid containers

OBS doesn't support ipv6 yet, but the memcached and passenger module in apache2 will bind on ipv6 if it's available. This cause problem to the obs-api rails app that results a broken rails app.

So that we need to disable ipv6 in the sid containers to avoid these issues while we do the tests.

Signed-off-by: Andrew Lee (李健秋) ajqlee@debian.org