Always return colors for single stat graphs

Addresses a change that requires single stat graphs to have an array of
colors, specifically  in the case of canned dashboards. An empty array
is acceptable but an undefined value will break the page.

Closes #2862
Closes #2841

canned/docker.json

@@ -56,6 +56,7 @@
           ]
         }
       ],
+      "colors": [],
       "type": "single-stat"
     },
     {
@@ -73,6 +74,7 @@
           ]
         }
       ],
+      "colors": [],
       "type": "single-stat"
     },
     {

canned/mesos.json

@@ -117,6 +117,7 @@
       "h": 4,
       "i": "0fa47984-825b-46f1-9ca5-0366e3220008",
       "name": "Mesos Master Uptime",
+      "colors": [],
       "type": "single-stat",
       "queries": [
         {

chronograf.go

@@ -581,6 +581,7 @@ type Cell struct {
 	Queries    []Query         `json:"queries"`
 	Axes       map[string]Axis `json:"axes"`
 	Type       string          `json:"type"`
+	CellColors []CellColor     `json:"colors"`
 }
 
 // Layout is a collection of Cells for visualization