containers: fix three cases' exit status

In iproute2 whose version is earlier than v3.0.0, ip
command doesn't support netns object. In this condition,
running three cases (netns_devices.sh, netns_devices2.sh
and netns_isolation.sh) will exit with TFAIL. It is not
suitable, so fix these cases' exit status with TCONF.

Signed-off-by: Xing Gu <gux.fnst@cn.fujitsu.com>
containers: change check_iproute() behavior in netns/netns_helper.h

Change check_iproute() behavior in order to improve
its expansibility.

Signed-off-by: Xing Gu <gux.fnst@cn.fujitsu.com>
containers: fix exit status in netns/netns_netlink.c

In iproute2 whose version is earlier than v2.6.34,
ip command doesn't support tuntap object. This leads
to the case (netns_netlink.c) failure with TFAIL status.
It is not suitable, so fix this case' exit status with
TCONF in the above condition.

Signed-off-by: Xing Gu <gux.fnst@cn.fujitsu.com>