Rudy/snmp table

Update to the snmp checks, modifying the way we query things.

Switching from doing snmpgets on the value specified in the configuration to
doing snmpwalks on domain specified by the user and gathering information from the results.

For completely specified objects, this doesn't change anything.
It allows for more powerful query: can query table whose size is not known at configuration time, can query object being in a table with multiple indexes, gather information for tags from the indexes or use information contained in other columns of the row as tags for the results we want.

I will also update the documentation in the documentation website with detailed examples.