Adds assert_node_exists()

This is similar to recently introduced assert_job_exists(). It will
throws an exception whenever a node is missing and further customize the
exception messages.

Replace an occurence of:

  if not self.node_exists(name):
      raise JenkinsException('node[%s] already exists' % (name))

By:

  self.assert_node_exists(name, 'node[%s] already exists')

Change-Id: Ia2e91843b353c02fb19290eadbbb83db8d2a3124
Merge "Adds assert_node_exists()"