sms: Introduce VTY-configurable minimum SMS validity period

This is meant as a safeguard against users or user equipment which
doesn't set a reasonable validity period.  Using this setting, the
SMSC administrator can set a minimum SMS validity period. Any SMS
submitted with lower validity period will be extended to that minimum.

Change-Id: I192528a6f9059d158fa12876a247d61bd7edaec8
Related: OS#5567
diff --git a/tests/test_nodes.vty b/tests/test_nodes.vty
index cb617fc..8ffb2bd 100644
--- a/tests/test_nodes.vty
+++ b/tests/test_nodes.vty
@@ -183,6 +183,7 @@
  queue max-failure 1
  database delete-delivered 1
  database delete-expired 1
+ validity-period minimum 1
  validity-period default 10080
 end