rename the SSL certificate related arguments

Calling it certifcate and private_key makes it more apparent what we're copying
about.

This is probably not the last change to this as there's the disjunct
from the underlying setting `plugin.ssl_client_private` and the
parameter name `certificate` to worry about at some point in the future.
more elegantly anchor mcollective::client::settings
Hack around resource containment

I think a cleaner work around this might end up being datacat with multiple
targets; so you would do something like

   datacat { "${homedir}/mcollective":
     alias => "mcollective::user",
   }

   define mcollective::client::setting($value) {
     datacat_fragment { "mcollective::client::setting $name":
       target => [ "mcollective::client", "mcollective::user" ],
       data   => { $name => $value },
     }
   }

   define mcollective::user::setting($value) {
     datacat_fragment { "mcollective::client::setting $name":
       target => [ "mcollective::user" ],
       data   => { $name => $value },
     }
   }

Then the config can be assembled at apply time from settings set by
mcollective::server::setting and mcollective::user::setting

Datacat doesn't currently support this, but adding it should be simple enough.