Azure: Improve script execution

[vinseon]

Multiple init scripts:

We currently combine all scripts (from an InstanceScript object) to execute them at once. This is actually required for 'initScript' only (from createInstance method) as we can only give one extension per type during the VM creation.

Advanced script upload:

Support advanced scripts by specifying and uploading file-based scripts into Azure environment (can be then used by the Azure extension). The script can be then executed from a regular cmdLine injection.

Windows compatibility:

Check deployment and script execution on Windows (using the same extension than Linux). Check combined script as well using the '&&' separator.

Encryption:

Allow to send encrypted scripts on VMs using specific credentials (Azure extension feature)