nuageinit: execute user-data scripts with explicit shell when needed

[olgeni]

Summary

Check if user-data scripts start with #!/bin/sh shebang and explicitly
invoke them with 'sh' to ensure proper execution. This handles cases
where the script file may not have execute permissions but is intended
to be run as a shell script.

[olgeni]

Closing this now that we have better version \o/