improve the snmp inspector and potentially the hardware pollsters

We've made https://blueprints.launchpad.net/ceilometer/+spec/monitoring-physical-devices landed in Icehouse release. It introduced the hardware pollsters, the inspector abstraction, and the snmp inspector implementation.

Just like Eoghan pointed out, there are still some improvement we can do about those:

1. generic mapping mechanism between the snmp OID and final sample in snmp inspector
Currently, the list of OIDs is interrogated quite statically in the snmp inspector. We need to find out how we can make the mapping of OID->inspector_method->sample more generic. The goal would be that new OID->sample mappings could be added to the mix *without* writing any new code(or writing as little code as possible). This could help when we want to add new pollsters using snmp, like the one bug #1308845 required.

2. management of snmp credentials
In the production env, snmpd is usually configured to have access control of credentials, and we can expect other inspector protocol might have credentials too(i.e. ssh, ipmi). Instead of putting the credential in plain-text url, we might need a way to store that credential somewhere else and use them programmatically in run-time. Barbican might be a good candidate.

3. Is the inspector data definition too snmp-ish?

4. other misc improvement
- bulkCmd support, https://bugs.launchpad.net/ceilometer/+bug/1286132