Improve the functionality of ec2-agent/vsphere-vm-agent: allow custom userdata

[gthao313]

Since we moved testsys from Bob to twoliter, the functionality of how we pass custom userdata to TestSys isn't appropriate.
It's trying to find a custom userdata toml file from Bob and parse it to acceptable format; however, it would fail due to the non-existent path at Bob.

https://github.com/bottlerocket-os/twoliter/blob/develop/tools/testsys/src/crds.rs#L128-L157