Raise an exception if no router_id provided

Raise an exception if no router_id provided

IF both service neutron-l3-agent and neutron-server are up,
but no router id configured in /etc/neutron/l3_agent.ini, an
exception will be raised on DB as "DBError: IntegrityError",
because the variable router_ids has a default '' value that
doesn't match the DB grammar.

    * Check router id is specified in _init_() of l3 when
      not using namespace.
    * Move part of checking config params actions to new function
      _check_config_params()
    * Add corresponding unit tests.

Closes-Bug: #1226366

Change-Id: I905f8a4061c5b250782e025681dfefcc41c8c03c