Change sshexec() to return a value

This pull request changes the sshexec() method to return a value of the last session closure.

```groovy
task example << {
  def uname = sshexec {
    session(remotes.localhost) {
      execute 'uname'
    }
  }
}
```

If more than one sessions are given, the result of the last defined session is returned (not last executed one).