Replace hardcoded provider for salt.control state

In case of using custom names for KVM nodes provided in
cookiecutter.json, after applying state salt.control it is not creating
VM's because provider is kvm01, kvm02, kvm03.

Change-Id: I4a7c1692f3e95fae6fc336ce605964cf78c47ea9
diff --git a/salt/control/cluster/infra_proxy_cluster.yml b/salt/control/cluster/infra_proxy_cluster.yml
index c82a947..d6cb317 100644
--- a/salt/control/cluster/infra_proxy_cluster.yml
+++ b/salt/control/cluster/infra_proxy_cluster.yml
@@ -13,10 +13,10 @@
           engine: virt
           node:
             prx01:
-              provider: kvm01.${_param:cluster_domain}
+              provider: ${_param:infra_kvm_node01_hostname}.${_param:cluster_domain}
               image: ${_param:salt_control_trusty_image}
               size: infra.proxy
             prx02:
-              provider: kvm02.${_param:cluster_domain}
+              provider: ${_param:infra_kvm_node02_hostname}.${_param:cluster_domain}
               image: ${_param:salt_control_trusty_image}
               size: infra.proxy