make infortrend plugin compatible with newer cmk versions

[mabezi]

The title explains the idea.

However, there are a few comments:

• I only tested the following checks: infortrend_chassis1, infortrend_disks1, infortrend_ldrives1
• There may still be a small error in infortrend_disks1, as a disk is displayed in our cmk instance that almost certainly doesn't exist.

I'd be happy if you mention me or my team (AG DSN) in the changelog :)