Cleans up the l3 agent manifest

This patch makes the following changes to the l3 agent manifest:

  * Adds a doc header
  * Removes the auth parameters
  * Removes the metadata IP parameter as its no longer used
  * Sets router and gateway IDs to undef values
  * Adds some tests

Change-Id: Ia36141fdb9433c629c7945cc1774b549bdff4c69