User can create clusters with duplicated names

[borisstoyanov]

Steps:

1. Create a cluster with name: c1
2. Create a second one with the same name

There's no unique constraint around that and user is able to create it. This diverges from the general bahaviour of clusters in Cloudstack. For example if the user goes and created a regular cluster in the infrastructure tab, he's being served with an uniqueness exception.