Add support in plugin config for accessing host ipc namespace.

Plugins might need access to host ipc namespace. A good usecase is
a volume plugin running iscsi multipath commands that need access to
host kernel locks.
Tested with a custom plugin (aragunathan/global-net-plugin-full) that's
built with `"ipchost" : true` in config.json. Observed using
`readlink /proc/self/ns/ipc` that plugin and host have the same ns.

Signed-off-by: Anusha Ragunathan <anusha.ragunathan@docker.com>

docs/extend/config.md

@@ -115,6 +115,9 @@ Config provides the base accessible fields for working with V0 plugin format
 
 	  options of the mount.
 
+- **`ipchost`** *boolean*
+   Access to host ipc namespace.
+
 - **`propagatedMount`** *string*
 
    path to be mounted as rshared, so that mounts under that path are visible to docker. This is useful for volume plugins.