Load balancer's withExistingVMs should create a backend right away, not guess later

[martinsawicki]

right now, withExistingVMs assumes a default backend to be created as needed at creation time or associates the VMs with the first backend found on the LB. This was the initial tentative logic. The proper way to do this though, now that backend infrastructure is supported is for each withExistingVMs call to create a separate backend right away for that set of VMs and remove the guesswork at creation time.
This also means an overload for withExistingVMs is needed that optionally takes a name for the new backend.

[martinsawicki]

Addressed in commit 56fc3a1