enb: add getter for number of cells of an eNB

In order to set up the channel emulator we need to calculate the total
number of RF channels that a eNB has from within a test.

Change-Id: If641993033eb5dcdc6e2d6afdc8426ed322241ca

src/osmo_gsm_tester/obj/enb.py

@@ -130,6 +130,9 @@ class eNodeB(log.Origin, metaclass=ABCMeta):
             return 1
         return 2
 
+    def num_cells(self):
+        return self._num_cells
+
 ########################
 # PUBLIC - INTERNAL API
 ########################