Update check-paloalto-A500.pl

In order for me to be able to check a little more stuff with your script I add some new check against my cluster of paloalto PA5050 

The modification that I made are 

model 
ha 
firmware 
TCP Sessions 
UDP Sessions 
UCMP Sessions 
Sessions
Merge pull request #5 from dufourl/patch-1

Update check-paloalto-A500.pl