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 and #6 (the dead helper).
