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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