Previously if subscriber was automatically created it got assigned
random MSISDN number. Make it optional (defaulting to previous behavior)
by adding following:
* new optional no-extension argument for subscriber-create-on-demand vty
command
* db unit tests
* vty test
Note: using the db made with new code might result in subscribers with
empty extension. Such subscribers cannot be deleted using old
code. Make sure not to mix db versions or manually fix it by editing
sqlite with external program.
Fixes: OS#1658
Change-Id: Ibbc2e88e4722b08854ebc631485f19ed56443cbb
e6052c4cc7