Fix libcontainer network support on rhel6

It seems that netlink in older kernels, including RHEL6, does not
support RTM_SETLINK with IFLA_MASTER. It just silently ignores it, reporting
no error, causing netlink.NetworkSetMaster() to not do anything yet
return no error.

We fix this by introducing and using AddToBridge() in a very similar manner
to CreateBridge(), which use the old ioctls directly.

This fixes https://github.com/dotcloud/docker/issues/4668

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)
Merge pull request #5115 from alexlarsson/fix-libcontainer-network-rhel6

Fix libcontainer network support on rhel6