snmp ip/mac addr conversion & oid index suffix support

[phemmer]

This adds 2 new features to the SNMP plugin:

1. Support for MAC address & IP address conversion. Both types of addresses, when returned as packed bytes, can be converted to human readable addresses.
2. Support for OID index suffixes. When joining multiple tables, sometimes the joined table has a slightly longer index as the table has a multiple records for each of the records in the origin table. The new oid_index_suffix allows stripping off the trailing part of the OID on the joined table so that the index will match.

closes #1755
closes #1729

Required for all PRs:

• CHANGELOG.md updated (we recommend not updating this until the PR has been approved by a maintainer)
• Sign CLA (if not already signed)

[phemmer]

I built this generator to make management of the mocked commands easier. Having to manually add the command & output to the test file was becoming a PITA.

[phemmer]

I split all the command mocking stuff out of test_snmp.go to keep the tests clean.

[phemmer]

@mattbakerau This should solve your use case (#1729). If you want to take a look at it, and/or try it out, any feedback would be great.

@mnctech This should also solve your use case (#1755). If you have any feedback, it would also be appreciated.

[sparrc]

LGTM, I can merge when you rebase the changes

[phemmer]

Rebased