3446 crm attribute

This address a potentially confusing situation where Puppet::Provider::Corosync needs `crm_attribute` installed, but doesn't say so. So, it would be up to the subclass to have `commands :crm_attribute => 'crm_attribute'` somewhere, which wouldn't make much sense in the context of that subclass, where `crm_attribute` could be unused.

Also expanded test coverage.