[rackspace|compute] Updated setup method to retry once on disconnect 

When using the `servers.bootstrap` with Ubuntu 12.04 images I consistently get the following exception

```
Net::SSH::Disconnect: connection closed by remote host
from /Users/krames/.gem/ruby/2.1.1/gems/net-ssh-2.9.0/lib/net/ssh/transport/packet_stream.rb:87:in `next_packet'
```
This apparently is being caused by the failure to do a nonblocking read during the authentication setup for Net::SSH. This PR catches that exception and attempts to retry the connection one more time.