Improve barclamp description

Also remove the tech preview part: this is specific to SUSE Cloud (so
should not be in a pebbles branch), and we might want to easily toggle
that off so a patch is good enough.
Merge pull request #3 from vuntz/hide-tech-preview

Improve barclamp description
Merge pull request #2 from mjura/release/pebbles/master

Making barclamp-ceph working