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 <anusha.ragunathan@docker.com>

docs/extend/config.md

@@ -117,6 +117,8 @@ Config provides the base accessible fields for working with V0 plugin format
 
 - **`ipchost`** *boolean*
    Access to host ipc namespace.
+- **`pidhost`** *boolean*
+   Access to host pid namespace.
 
 - **`propagatedMount`** *string*