Re-add the check cmd for installation, fix ceph-deploy path in mds.pp

This re-adds the "unless" condition in init.pp that will only install
packages if packages having "ceph" in their name are not there already.
Partially inspired by Bastian Ballman's commit in

https://github.com/balle/puppet-cephdeploy/commit/71233e6b9f2d169c1877012abdfaa0e89db4d76c

I have adapted his changes to fit my fork of puppet-cephdeploy, but the
outcome will be the same.
documentation update
Make repository installation controllable via $ceph_install_repositories

Some people may want to use separate package repositories instead of
those provided by Inktank; for such cases, make the installation of
repositories controllable using a separate parameter, which is named
ceph_install_repositories.