fix_snmp hard-coded community "public". The driver implements napalm-device-types 4.13's configure_snmp_agent as a whole, because over REST a temporary SSH session has to run the config-mode CLI: community (read-only), contact and location from the caller, saved and verified in show snmp-server. The agent is built in and the switch filters nothing for its management plane; the steps say so instead of pretending. A value that would break the CLI is refused. The unused _detect_management_vlan goes with it. BREAKING CHANGE: run_device_action is removed. Part of #12. Closes #6.