fuel-agent: improve compliance with py3

[Alexander Gordeev]

py3 readiness is only proven by unit-tests and this is not enough from practical point of view.

Fuel is quite ready to be switched to next ubuntu release (where py3 is the default version of python), that means that bootstrap image will be built with py3 in mind, therefore fuel-agent must be purely operable under py3 too.

That could act as a blocker for mos-xenial blueprint.

In particular, every file write should be handled in a specific way which py3 requires. Perhaps, there's plenty of other gaps in fuel-agent code.

Actual results: may vary, since ASCII is compatible with UTF-8. Use of data out of ASCII rage may lead to encode errors or something similar (these facts are not accurate, that wasn't checked too much, just a guess)
Expected results: not only ASCII, but also unicode data should be processed without any flaws.