Snmp checks

Agent check to allow SNMP polling
Add pysnmp and pysnmp-mibs dependency
Based on the list of metrics specified in the yaml file, will poll the different snmp enabled device.
Support SNMP v1,v2 and v3
SNMP Counter metrics are transformed as a rate and gauge are reported as gauge.

Tests are not passing due to pylint errors fixed in another PR #923 

Will that happen a lot? -> for most users it won't happen.
It will happen if they are using some non-standard mibs that are creating their own type (ie not counter or gauge ) and that we don't know how to report.
In that case we're not going to report them. I don't think we want to error in that case but it would be weird to see that everything is OK and still not seeing the metrics you defined in the YAML file.