Use unique incrementing value for BTS CellId

Change-Id: If4559b945a3e71f3a36fd7ac760cb094278f6b39
diff --git a/src/osmo_gsm_tester/bts_octphy.py b/src/osmo_gsm_tester/bts_octphy.py
index eb1092c..bc93a3a 100644
--- a/src/osmo_gsm_tester/bts_octphy.py
+++ b/src/osmo_gsm_tester/bts_octphy.py
@@ -31,6 +31,7 @@
     pcu_sk_tmp_dir = None
     values = None
     lac = None
+    cellid = None
 
     BIN_BTS_OCTPHY = 'osmo-bts-octphy'
 
@@ -161,6 +162,8 @@
         config.overlay(values, config.get_defaults('osmo_bts_octphy'))
         if self.lac is not None:
             config.overlay(values, { 'location_area_code': self.lac })
+        if self.cellid is not None:
+            config.overlay(values, { 'cell_identity': self.cellid })
         config.overlay(values, self.conf)
         self.dbg(conf=values)
         return values
@@ -171,4 +174,7 @@
     def set_lac(self, lac):
         self.lac = lac
 
+    def set_cellid(self, cellid):
+        self.cellid = cellid
+
 # vim: expandtab tabstop=4 shiftwidth=4