Use unique incrementing value for BTS CellId

Change-Id: If4559b945a3e71f3a36fd7ac760cb094278f6b39
diff --git a/src/osmo_gsm_tester/resource.py b/src/osmo_gsm_tester/resource.py
index 9571131..1bacd38 100644
--- a/src/osmo_gsm_tester/resource.py
+++ b/src/osmo_gsm_tester/resource.py
@@ -213,6 +213,9 @@
     def next_lac(self, origin):
         return self.next_persistent_value('lac', '1', schema.uint16, lambda x: str((int(x)+1) % pow(2,16)), origin)
 
+    def next_cellid(self, origin):
+        return self.next_persistent_value('cellid', '1', schema.uint16, lambda x: str((int(x)+1) % pow(2,16)), origin)
+
 class NoResourceExn(log.Error):
     pass