Fix PREFIX in build_push_update_images.sh (#23022)
Fix PREFIX in build_push_update_images.sh (#23022)

Previously, you couldn't use PREFIX to override the docker.io prefix. Also,
the prefix wasn't used when the script modified the YAML files. Now, you can
use PREFIX to specify the whole prefix (e.g. use a different registry) and
the YAML files are updated properly.

Also, the VERSION could only contain digits and dots. Now it can be any
string (e.g. something-1.0.0).
smawson/istio,istio/istio,smawson/istio,smawson/istio,smawson/istio,istio/istio,istio/istio,smawson/istio,istio/istio,smawson/istio,istio/istio