– Create a new user in ESXi with no access privilege, you need to login to the ESXi directly to do that.

– Enable SSH, and add nagios user to root group:
# vi /etc/group
root:x:0:root,nagios

– Check from the command line, if it works
./check_esxi_hardware.py --host https://esxihost:5989 --user file:credentials.txt --pass file:credentials.txt
OK - Server: Cisco Systems Inc.....

– Configure the credentials files to use the nagios user credentials.