Add snmp_trap input plugin

[reimda]

Required for all PRs:

• [ x ] Signed CLA.
• [ x ] Associated README.md updated.
• [ x ] Has appropriate unit tests.

closes #4377

[reimda]

to do:

• Mock OID name resolution so unit tests pass even when snmptranslate isn't available
• Add v1 traps. Version 2c traps seem to work now.
• Format values better, possibly using the same mechanism as the snmp plugin or by adding support for textual convention in the MIB.
• Check on upstream support for inform.

[tesibelda]

Great job reimda. Some suggestions:

• it would be useful to have a snmp_mib tag added. It would be helpfull for post processing filtering of these points (say with kapacitor). When using a large amount of MIBs, some of them might use the same string for snmp_trap.
• it would be useful to have a snmp_host tag added. It would include the DNS resolved name of the IP address of device sending the trap. This would help identify the device having the issue reported by the trap.

[reimda]

Thanks for the feedback @tesibelda

* it would be useful to have a snmp_mib tag added ...

I added this as a tag called trap_mib to match the existing tag trap_name.

* it would be useful to have a snmp_host tag added. It would include the DNS resolved name of the IP address of device sending the trap ...

Since we already have the numeric IP address, I added it as a tag called source. Other inputs use the name "source" for this so it makes sense to follow convention. I agree a DNS name could be more useful than the IP but I'd like to wrap up this PR. Adding a setting to do reverse DNS sounds like a great follow-on feature.