ctrl: Introduce cmd SET subscriber.create <imsi>

Create a new subscriber from CTRL, similar to VTY command
"imsi IDENT create".

On success SET_REPLY contains the ID of the newly created subscriber.

Related: SYS#5993
Change-Id: Id1b760cd07712245a0eeabaac7891bce93c1fe8e
diff --git a/tests/test_subscriber_errors.ctrl b/tests/test_subscriber_errors.ctrl
index 0c5b587..425b0df 100644
--- a/tests/test_subscriber_errors.ctrl
+++ b/tests/test_subscriber_errors.ctrl
@@ -108,3 +108,9 @@
 
 GET 48 subscriber.by-id-0x0123.info
 ERROR 48 Invalid value part of 'by-xxx-value' selector.
+
+SET 49 subscriber.create zzz
+ERROR 49 Invalid IMSI value.
+
+SET 50 subscriber.create 901990000000001
+ERROR 50 Subscriber already exists.