ansible: chanage production gsm_modems to 3

In the production setup, there are only 3 valid modems.
Moving the variable into the host_vars so they can have
different amount of modem between rnd & prod.

Change-Id: Ic51a14a339bb9be50504a27a724f6f428ee7304f

ansible/host_vars/osmo-gsm-tester-prod.yml

@@ -4,3 +4,6 @@ udhcpd_static_leases:
     ip: 10.42.42.53
   - mac: 00:02:95:00:5a:a4
     ip: 10.42.42.54
+
+# how many modems are connected via a quadmodem?
+gsm_modems: 3

ansible/host_vars/osmo-gsm-tester-rnd.yml

@@ -2,3 +2,6 @@
 udhcpd_static_leases:
   - mac: 00:02:95:00:41:b3
     ip: 10.42.42.53
+
+# how many modems are connected via a quadmodem?
+gsm_modems: 4

ansible/setup-gsm-tester.yml

@@ -17,8 +17,6 @@
         - curl
   roles:
     - name: gsm-tester
-      # how many modems are connected via a quadmodem?
-      gsm_modems: 4
       tags:
         - gsm-tester