Add a new type ClusterData

This will be used to hold all the disparate uses of the cluster data:
we have either tuples with these four elements, or functions taking
these four arguments, etc.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Balazs Lecz <leczb@google.com>

Ganeti/HTools/Loader.hs

@@ -36,6 +36,8 @@ module Ganeti.HTools.Loader
     , commonSuffix
     , RqType(..)
     , Request(..)
+    , ClusterData(..)
+    , emptyCluster
     ) where
 
 import Data.List
@@ -74,6 +76,18 @@ data RqType
 data Request = Request RqType Group.List Node.List Instance.List [String]
     deriving (Show)
 
+-- | The cluster state.
+data ClusterData = ClusterData
+    { cdGroups    :: Group.List    -- ^ The node group list
+    , cdNodes     :: Node.List     -- ^ The node list
+    , cdInstances :: Instance.List -- ^ The instance list
+    , cdTags      :: [String]      -- ^ The cluster tags
+    } deriving (Show)
+
+-- | An empty cluster.
+emptyCluster :: ClusterData
+emptyCluster = ClusterData Container.empty Container.empty Container.empty []
+
 -- * Functions
 
 -- | Lookups a node into an assoc list.