Add UCD-SNMP-MIB.yaml

I converted http://www.net-snmp.org/docs/mibs/UCD-SNMP-MIB.txt as yaml. 

There are commonly used oids in UCS-SNMP-MIB name space. For example, `memAvailSwap` for memory usage or `laTable` for load average. I think this definition will be useful for users, so how do you think? 

Detail of UCD-SNMP-MIB is here: http://www.net-snmp.org/docs/mibs/ucdavis.html