Use unique incrementing value for BTS CellId

Change-Id: If4559b945a3e71f3a36fd7ac760cb094278f6b39
diff --git a/src/osmo_gsm_tester/suite.py b/src/osmo_gsm_tester/suite.py
index 9f9421e..fc4930d 100644
--- a/src/osmo_gsm_tester/suite.py
+++ b/src/osmo_gsm_tester/suite.py
@@ -345,6 +345,7 @@
     def bts(self, specifics=None):
         bts = bts_obj(self, self.reserved_resources.get(resource.R_BTS, specifics=specifics))
         bts.set_lac(self.lac())
+        bts.set_cellid(self.cellid())
         self.register_for_cleanup(bts)
         return bts
 
@@ -376,6 +377,11 @@
         self.log('using LAC', lac)
         return lac
 
+    def cellid(self):
+        cellid = self.resources_pool.next_cellid(self)
+        self.log('using CellId', cellid)
+        return cellid
+
     def poll(self):
         if self._processes:
             for process in self._processes: