Add AllowedValues constraint to Rackspace::Cloud::LoadBalancer

The Rackspace Cloud Load Balancer service only allows certain
values for the algorithm property. This patch adds constraints
so that users get validation feedback prior to creating the
resource.

Closes-Bug: #1328654
Change-Id: Id24b426b1d3e49b09b3e468ea9e9675c05eaa160
Merge "Add AllowedValues constraint to Rackspace::Cloud::LoadBalancer"