containers/netns: enable IPv6 address autoconfiguration for veth interfaces

In some linux distributions, such as RHEL7U0RC1, when we create a veth pair
by "ip link add type veth", the ipv6 address for veth interfaces won't be
automatically generated, so the "par_chld_ipv6" test case in containers test
will fail. This is because in these distirbutions, the "/proc/sys/net/ipv6/
conf/veth*/disable_ipv6" is true for veth interfaces. Please see kernel
"Documentation/networking/ip-sysctl.txt" for more detailed information.

In this patch, we will enable IPv6 address autoconfiguration for veth interfaces
if disabled.

Signed-off-by: Xiaoguang Wang <wangxg.fnst@cn.fujitsu.com>