Add VTY commands to set error ctr thresholds

osmo-trx will validate over time that those thresholds are not reached.
If they are reached, osmo-trx will die. As a result, osmo-bts-trx will
notice and will end up notifying the BSC about it (for instance because
it will also restart its process).

For instance:
"""
ctr-error-threshold rx_drop_events 2 minute
ctr-error-threshold rx_underruns 10 second
"""

In those cases above, osmo-trx will die if rate_ctr rx_drop_events went
to a value higher than 2 per minute, or it will die to if rx_underruns
went higher than 10 per second.

Change-Id: I4bcf44dbf064e2e86dfc3b8a2ad18fea76fbd51a
6 files changed