Tested using global-net-plugin-ipc which sets PidHost in config.json.
Plugins might need access to host pid namespace. Add support for that.
Tested using aragunathan/global-net-plugin-ipc which sets "pidhost" in
config.json. Observed using `readlink /proc/self/ns/pid` that plugin and
host have the same ns.
Signed-off-by: Anusha Ragunathan <anusha.ragunathan@docker.com>