Set launch_index to right value

Fix bug 1212648

When boot multiple instances in one request, the launch_index
was not increased.

The fix was increase the launch_index one by one for each instances
in one request.

Conflicts:
	nova/compute/api.py
	nova/tests/compute/test_compute.py

Change-Id: I27dde8cfae52853d33277c3d8e0e65c91bc6280b
(cherry picked from commit c8f0a545bfafdc72bae0a43df78ae51835e35232)