SNMP read-write user should not be removed from node/cluster.cfg

The security.administrators configuration setting from node/cluster.cfg should always include the snmp user (the user that has SNMP read-write permissions). Removing the snmp user is not supported, because it will prevent you from changing settings dynamically through snmpset and from rebooting the cluster from the web console.

E.g.:

- Correct setting: 

security.administrators = {'admin':'passzzz', 'snmp':'passzzz'}

- Incorrect setting:

security.administrators = {'admin':'passzzz'}