make helpers permanent to defend against Chef cache wipe

If the maintenance mode handlers got installed, and then the node reboots and the first `chef-client` in `crowbar_join` fails, crowbar_join wipes `/var/cache/chef`, and that broke loading of the handlers via
`/etc/chef/client.rb` on `chef-client` startup, since `maintenance_mode_helpers.rb` would then be missing.

To defend against this, we additionally install the helpers into a permanent location outside the cache (`/var/chef/libraries`) at the same time the handlers are installed.

Supercedes https://github.com/crowbar/barclamp-pacemaker/pull/55.