Get rid of treetop dependency

The lsi ohai plugin depends on the treetop gem. While it is easy to install ruby-treetop Debian package on nodes with system-installed chef-client, it is a PITA to deploy the gem as a chef_gem on Omnibus-installed chef/cing nodes, esp. on machines without internet access.

Best way forward would be to drop the treetop dependency. Maybe the lsi ohai plugin has been superseded by the storcli plugin anyhow?