make retrieval of IMEISV configurable
Follow-up to #3189, where check-imei-rqd was made configurable. IMEISV is still missing.
We already have flags that trigger retrieving of IMEI / IMEISV in the course of the VLR's FSMs.
We also have unit tests that go through these scenarios and verify they work.
What we still need is to switch them on: add vty config to set vlr_instance.cfg values: