adding functionality to check for primary netscaler in a cluster

Hello,

Submitting a pull request as we found it might be useful for everyone else to benefit from determining which netscaler in your environment is primary, especially if you have multiple netscalers in a cluster.

Usage:
./nsnitrocmd.py --lbip 1.2.3.4 --user nsroot --password nsroot --getprimarylb

The nice thing is you only have to hit one of your netscalers to determine which one is primary, the result you get back from the above script is the ip address of the primary netscaler.  If you only have one netscaler than, this probably won't benefit you much...