NMS-4265:  Clean-up Dell events definitions

Hi guys,

this PR clean up and updates the Dell events definitions for related snmp traps.

In brief, what I did is:

1- Split the Dell.events.xml file into smaller ones (one file per MIB)
2- update these files with the definitions from ftp://ftp.us.dell.com/sysman/DCMIB65.zip (latest version of the MIBs available)

This should solve issues NMS-4264, NMS-4266, and NMS-6756. So, it should also solve issue NMS-4265.

Best regards,

Cyrille