fix mscpool cleanup (fixes LCLS tests)

osmo-bsc takes a while to notice that a connected MSC is no longer connected.
Once the mscpool tests have run, the additional msc 1 and msc 2 still linger
around even though the BSSMAP link is no longer served by the bsc-tester.

The easiest way to ensure that only expected MSCs are contacted is to set
'no allow-attach' for each MSC that should not be in use.

So, the default setup is 'allow-attach' on msc 0, and 'no allow-attach' on mscs
1 and 2. In f_init(), allow attach on those MSCs indicated by the nr_msc
amount. The entire vty transaction to configure attach/no attach for all three
MSCs takes about 4 micro seconds in my test setup, so it is fine to do this
during f_init() for each BSC test.

After this, tests running after the MSC pooling tests (the LCLS tests) no
longer round-robin their subscribers across disconnected MSCs.

NOTE: it would be good to somehow detect more reliably in osmo-bsc that an MSC
is gone and not use it anymore. That is however not so trivial. To get the LCLS
tests back online, this is a workaround to avoid that complexity for now.

Change-Id: I02ad58ed7d0d0aac61e393b415e09c6c5c8a70ca
1 file changed