Allow resetting the BSSMAP link from VTY, for BSC_Tests.ttcn.
In the field, detecting that an MSC is lost is done by getting three
connection failures in a row. For the BSC_Tests, it is easier to just
provide a VTY command to reset an MSC's link status.
I want to add tests that verify the stat items reflecting the MSC
connection status. To be able to run a test expecting fewer connected
MSC after a test that launched more MSCs requires the links to be reset.
Related: SYS#5542
Related: Ice3056dc46c94f9399f8379db7aeb7193782f2f2 (osmo-ttcn3-hacks)
Change-Id: I1975941b790d2b30d0904d41e456220cba26ecff