Add an option to prevent putting the shim in a new mount namespace

This is needed for users on kernel older than 3.18 so they can avoid EBUSY
errors when trying to unlink, rename or remove a mountpoint that is present in
a shim namespace.

Signed-off-by: Kenfe-Mickael Laventure <mickael.laventure@gmail.com>

docs/ops.md

@@ -212,4 +212,7 @@ The linux runtime allows a few options to be set to configure the shim and the r
 	no_shim = false
 	# display shim logs in the containerd daemon's log output
 	shim_debug = true
+	# do not put the shim in its own mount namespace
+	# (this only need to be set on kernel < 3.18)
+	shim_no_newns = true
 ```