Replace ip netns exec with nsenter

[awh]

Fixes #458. All smoke tests pass (400_weave_local_test.sh necessitated updating the Vagrantfiles to Ubuntu 14.10) but nevertheless quite invasive - suggest merging after 0.10.0?

[rade]

• The repetition of nsenter --net=$PROCFS/$NETNS/ns/net is a bit tiresome. Yes, the same was true for ip netns exec $NETNS, but at least that was shorter.
• $NETNS worked as a var name for ip netns exec $NETNS, but is somewhat misleading for use in $PROCFS/$NETNS. Could we just use $CONTAINER_PID instead?
• We need to check what, if any, new version constraints this imposes on the docker host's kernel. While nsenter itself is only a user space tool, it does depend on kernel features, so the questions is whether it depends on more recent features than ip netns exec.

[awh]

The repetition of nsenter --net=$PROCFS/$NETNS/ns/net is a bit tiresome. Yes, the same was true for ip netns exec $NETNS, but at least that was shorter.

Abstracted out to a function.

$NETNS worked as a var name for ip netns exec $NETNS, but is somewhat misleading for use in $PROCFS/$NETNS. Could we just use $CONTAINER_PID instead?

Done.

We need to check what, if any, new version constraints this imposes on the docker host's kernel. While nsenter itself is only a user space tool, it does depend on kernel features, so the questions is whether it depends on more recent features than ip netns exec.

I've smoke tested this on Ubuntu Precise 12.04/3.5.0-54-generic and it works (modulo #585 and 400_weave_local_test.sh).

[awh]

Fixed with_container_netns comment.