Add pid host support
Add pid host support

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 <ab61560b6c027d679611acf9b4b553fb22d61c05@docker.com>

Zebrilee/cli,allencloud/cli,docker/cli,thaJeztah/cli,ashfall/cli,thaJeztah/cli,redpanda/cli,cpuguy83/cli,cpuguy83/cli,allencloud/cli,ashfall/cli,riyazdf/cli,e11137/cli,e11137/cli,dnephin/cli,Zebrilee/cli,docker/cli,dnephin/cli,redpanda/cli,riyazdf/cli