updated carbon_sync to be able to handle configs without default 'main' ...

Ran into a little issue using carbon-sync.  I was specifying a carbonate.conf without a "main" cluster.  It was resulting in the ssh_user method in config.py throwing the following error:

```$ carbon-sync -s source -c carbonate.conf -C old_cluster
Cluster 'main' not defined in carbonate.conf```

This looks to be a simple fix.  Tests pass.  Great tool!