DISKUSED - add warning when volume is missing

When volume isn't found, then change state to warning.
DISKSUMMARY - more information to performance data

Add values of total, active, spare and reconstructing disks to performance data.
UPTIME - add performance data

Add uptime as performance data in seconds
Add new tested devices as comment

Add IBM N6240 and FA2050 identification as tested device with this plugin.
Merge pull request #8 from svamberg/netapp_new_features

Netapp new features