Drop the interpreter for execution in the service file

[rjschwei]

Description

The waagent script is executable and has the interpreter set. Drop the use of the Python interpreter to execute the file from the systemd unit file. We achieve unbuffered I/O by setting the PYTHONUNBUFFERED environment variable instead. This makes it easier for distribution packaging as packagers will only have to update the pathto the interpreter used on the given distro in one file.

Issue #

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[Klaas-]

I suggested drop the python execution a couple of years ago, MSFT were not interested :D
PR: #2729
Original issue: #2728

[narrieta]

@rjschwei - Extensions inherit the environment of the Agent. This change ended up breaking the Microsoft.CPlat.Core.LinuxPatchExtension extension since it changes the timing of the extension. Although this is actually a bug in the extension and a fix is currently being deployed, we do not know if other extensions have similar issues, plus not all VMs may be configured to take extension updates.

The PR makes a change that applies to distros other than SUSE, so we consider it to be high risk. Probably it may be worth considering whether this change can be undone in SUSE, but if it cannot, we suggest updating this PR to apply only to SUSE.