[RFE] SNMP driver should allow for different community for read/write operaions

[Ilya Etingof]

Ironic ships SNMP driver for hardware management purposes. SNMPv1/v2c requires the use of so-called community name string acting as a shared secret between SNMP manager (Ironic) and SNMP agent (hardware).

Conventionally, SNMP agents are frequently configured to use different SNMP community strings for read and write operations. The motivation behind that is privilege separation.

With Ironic SNMP driver it is not currently possible to configure SNMP manager to use different SNMP community names for SNMP GET and SET commands. This bug is to fix that, possibly by introducing new node configuration options like `snmp_read_community` and `snmp_write_community`.