[Ingest Manager] Use EPR's Image.path which replaces Image.src

[jfsiii]

Kibana changes required for elastic/package-registry#629

The registry-root-relative Image.src is renamed Image.path and Image.src is now package-root-relative / as-authored

e.g. from https://github.com/elastic/package-registry/pull/629/files#diff-977f05e7b5b1fb1f9d46aa963957b983L22-R23

-     "src": "/package/example/1.0.0/img/kibana-iptables.png",
+     "src": "/img/kibana-iptables.png",
+     "path": "/package/example/1.0.0/img/kibana-iptables.png",

Update code to use Image.path if both are present or fall back to Image.src if that's the only property available

[elasticmachine]

Pinging @elastic/ingest-management (Team:Ingest Management)

[ph]

@jfsiii Can you take a look at it? changes seem local and it would be good to have it soon.