Add scenario test for load balancer

This test checks basic load balancing.

The following is the scenario outline:
  1. Create an instance
  2. SSH to the instance and start two servers
  3. Create a load balancer
  4. Send 10 requests to a floating ip associated with a VIP
     and check that they are shared between the two servers
     and that both of them get equal portions of the requests

bp lbaas-scenario-tests

Change-Id: Ia864a6f532ea58747f04658eb38535ee06a303cb
The rescue tests requires neutron resources

test_rescued_vm_add_remove_security_group and
the test_rescued_vm_associate_dissociate_floating_ip does not makes
sense if the instance does not have a network connection.

'network' and 'subnet' would be sufficient for the security_group test
case, but the router is required for floating_ip test case.

Closes-Bug: #1274121
Change-Id: Ic8e70d5e549573213b2930d88b5d7da5cd4cf855
Merge "Add scenario test for load balancer"
Merge "The rescue tests requires neutron resources"
Merge branch 'master' of https://github.com/openstack/tempest