Version 2.17 enabled HTTPS certificate validation by default. Prior to that, the vSphere plugin always disabled SSL certificate validation, which was a security flaw as it opened up Jenkins to a man-in-the-middle attack.
FYI the security fix there was to change a "disable security check" boolean that was hard-coded to "true" into a user-configurable field that defaulted to "false" for anyone upgrading.
So, while 2.17 fixed that security flaw by enabling SSL validation, you can turn it off again through the configuration UI which, while that leaves you an in insecure state (just like 2.16 and earlier), it means that the way the plugin connects is exactly the same as it was in 2.16 and earlier.
So, short term, you can tick the "Disable SSL Check" box (Manage Jenkins -> Configure System, scroll down to the vSphere Cloud section).
Long term, I think you're going to have to fix your certificate errors so that you don't have to disable security features to make things work...