Make propagated mount persist outside rootfs

This persists the "propagated mount" for plugins outside the main
rootfs. This enables `docker plugin upgrade` to not remove potentially
important data during upgrade rather than forcing plugin authors to hard
code a host path to persist data to.

Also migrates old plugins that have a propagated mount which is in the
rootfs on daemon startup.

Signed-off-by: Brian Goff <cpuguy83@gmail.com>
Upstream-commit: df4e570ff4
Component: cli

components/cli/docs/extend/config.md

@@ -118,6 +118,8 @@ Config provides the base accessible fields for working with V0 plugin format
 - **`propagatedMount`** *string*
 
    path to be mounted as rshared, so that mounts under that path are visible to docker. This is useful for volume plugins.
+   This path will be bind-mounted outisde of the plugin rootfs so it's contents
+   are preserved on upgrade.
 
 - **`env`** *PluginEnv array*