Added Disk Capacity in cluster-data-model

Fetched information of total disk capacity from nova and added a new
resource 'disk_capacity' to NovaClusterModelCollector cluster. Also a
new resource type 'disk_capacity' was added to ResourceType.
https://bugs.launchpad.net/watcher/+bug/1553124

Change-Id: I85750f25c6d2693432da8e5e3a3d0861320f4787
Closes-Bug: #1553124

watcher/decision_engine/model/resource.py

@@ -21,6 +21,7 @@ class ResourceType(Enum):
     cpu_cores = 'num_cores'
     memory = 'memory'
     disk = 'disk'
+    disk_capacity = 'disk_capacity'
 
 
 class Resource(object):