Reads a remote file into a string.

def ReadRemoteFile(remote_file_path, hostname, ssh_key):
  """ Reads a remote file into a string. """
  cmd = 'sudo cat %s' % remote_file_path
  exit_code, output = RunCommandOnHost(cmd, hostname, ssh_key)
  if exit_code:
    raise IOError('Can not read remote path: %s' % (remote_file_path))
  return output