Provide an user error and exit when script is run as non-root for OVA deployments of APIC

[subindani]

I am seeing that few customers are running postmortem as apicadm user instead of rootin OVA deployments of APIC.

Could we please throw an error and exit from the script when they run it as non-root user, when the -ova flag is passed as an option. This would display an error to the user and they can take the corrective action, which is to run as root. This also decreases the ping-pong between us and customer when collecting the logs.