default server in remote filebuckets

With the path parameter set to false, the server defaults
to Puppet[:server]. This allows to use a remote filebucket without
syncing the servername there with the one used on the config file.

To use the default server, this manifest can be used:
filebucket { main: path => false }

A related bug report is:
http://projects.reductivelabs.com/issues/2456