orchestrator: inner-tree path can be a list.

Shared trees are supported by specifying the inner-tree as a list in the
mcombo file.  This change enables that work.

Bug: None
Test: manual, unittests pass

Change-Id: I161d707d0aada66d134b49b158bf538f0e2a2572
diff --git a/orchestrator/multitree_combos/test.mcombo b/orchestrator/multitree_combos/test.mcombo
index 3ad0717..d601b7d 100644
--- a/orchestrator/multitree_combos/test.mcombo
+++ b/orchestrator/multitree_combos/test.mcombo
@@ -1,16 +1,16 @@
 {
     "lunchable": true,
     "system": {
-        "tree": "inner_tree_system",
+        "inner-tree": "inner_tree_system",
         "product": "system_lunch_product"
     },
     "vendor": {
-        "tree": "inner_tree_vendor",
+        "inner-tree": "inner_tree_vendor",
         "product": "vendor_lunch_product"
     },
     "modules": {
         "com.android.something": {
-            "tree": "inner_tree_module"
+            "inner-tree": ["inner_tree_module", "sc-common"]
         }
     }
 }