nitb_netreg_mass: Allow a suite to configure num subscriber

Remove hardcoded configuration and replace it with a reservation in
the suite.

Change-Id: Ic9fd560c2924731e4fd6eea5aaf3ad565cb4ef52
diff --git a/suites/nitb_netreg_mass/register_default_mass.py b/suites/nitb_netreg_mass/register_default_mass.py
index ede2b74..e7c91da 100644
--- a/suites/nitb_netreg_mass/register_default_mass.py
+++ b/suites/nitb_netreg_mass/register_default_mass.py
@@ -9,6 +9,7 @@
 nitb = suite.nitb()
 bts = suite.bts()
 ms_driver = suite.ms_driver()
+modems = suite.all_resources(suite.modem)
 
 print('start nitb and bts...')
 nitb.bts_add(bts)
@@ -16,9 +17,10 @@
 bts.start()
 wait(nitb.bts_is_connected, bts)
 
-# Configure all MS that the MS driver knows about.
-for ms in ms_driver.ms_subscribers():
-    nitb.subscriber_add(ms)
+# Configure all MS that are available to this test.
+for modem in modems:
+    nitb.subscriber_add(modem)
+    ms_driver.subscriber_add(modem)
 
 # Run the base test.
 ms_driver.run_test()