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.