netns : allow netns probe to work within a containerized agent
netns : allow netns probe to work within a containerized agent

As mentioned in https://github.com/docker/docker/pull/17034
in order to follow the netns corresponding file when it gets bind mounted,
the netns directory bind mounted in the skydive agent container with a
docker volume option needs to have the "shared" propagation flag set.

By default, the propagation flag of a docker volume is "private", not
"shared".

This works since the usual source (/var/run/netns) also has the
"shared" propagation flag. Otherwise, one has to set it on the
source before starting the skydive agent container.

Change-Id: Ic321681269aba267a51bc0782f68683660c9eca0
Reviewed-on: https://softwarefactory-project.io/r/6397
Reviewed-by: Nicolas PLANEL <2ba60255afd05ede961f6b959482fc724759f990@redhat.com>
Reviewed-by: Sylvain Baubeau <b0a7dc08a3f38d713e107103b0a85b0badc14b84@redhat.com>
Tested-by: Sylvain Baubeau <b0a7dc08a3f38d713e107103b0a85b0badc14b84@redhat.com>
Workflow: Sylvain Baubeau <b0a7dc08a3f38d713e107103b0a85b0badc14b84@redhat.com>
Tested-by: Jenkins CI <d95b56ce41a2e1ac4cecdd398defd7414407cc08@softwarefactory-project.io>

skydive-project/skydive,safchain/skydive,safchain/skydive,redhat-cip/skydive,safchain/skydive,lebauce/skydive,lebauce/skydive,skydive-project/skydive,lebauce/skydive,lebauce/skydive,skydive-project/skydive,lebauce/skydive,skydive-project/skydive,skydive-project/skydive,lebauce/skydive,safchain/skydive,redhat-cip/skydive,safchain/skydive,safchain/skydive,safchain/skydive,lebauce/skydive,safchain/skydive,skydive-project/skydive,redhat-cip/skydive,lebauce/skydive,skydive-project/skydive