order of startup script execution is random

[fsiegmund]

If there is more than one startup script in the instance metadata, the order of execution is not deterministic, even so in line 349 of metadata_scripts/GCEMetadataScripts/main.go it says: "This ordering matters. URL is last on Windows, first otherwise.".

This is caused be parseMetadata returning a map instead of an array of key value pairs. Having more than one startup script is quite useful, the code supports this already.