Default SNMP parameter changes

max-repetitions = 10 is the default of net-snmp utils according to
http://net-snmp.sourceforge.net/docs/man/snmpbulkwalk.html

retries = 3 is the default of gosnmp:
https://godoc.org/github.com/soniah/gosnmp#pkg-variables

Could deal with some parts of the performance issues reported
by #1665