VTY: add show bts failure report

Save OML failure reports from each BTS. Add a VTY command to display them
conveniently and optionally clear the list.

OsmoBSC> show bts 0 fail-rep
[2020-03-23 14:51:22] Type=processing failure, Severity=minor failure, Probable cause=Manufacturer specific values: Software warning, Additional text=test message sent from VTY
[2020-03-23 14:51:19] Type=processing failure, Severity=minor failure, Probable cause=Manufacturer specific values: Software warning, Additional text=test message sent from VTY

Related: OS#1605
Change-Id: I18aa17a721cd5eb1c98926dc2367229c0a50bc78
4 files changed