Evgeny Golyshev

The `check_if_variable_is_set` function is invoked at the top of each script from the `bootstrap` directory. It doesn't make any sense, so get rid of the practice.

`deb.sh` is very common name. The helper is closely connected with [debootstrap](https://wiki.debian.org/Debootstrap), so I suggest we will rename it into `debootstrap.sh`.

Pieman has got two helpers, `others.sh` in `misc.sh`, which duplicate each other. It's necessary to move the contents of `others.sh` into `misc.sh` and get rid of `others.sh`.

Now Pieman requires Python 3.5 or higher. It's not necessary to support such an old version, so we can increase it to leverage some "tasty" features from fresh versions of...

# Please make sure all below checkboxes in the Mandatory section have been checked before submitting your PR (also don't forget to pay attention to the Depending on Circumstances section)...

Closes #239 --- # Please make sure all below checkboxes in the Mandatory section have been checked before submitting your PR (also don't forget to pay attention to the Depending...

Now it’s only possible to run `pieman.sh` when the root of the source tree of Pieman is the working directory.

The `BUILD_DIR` parameter should be immutable in the Docker container, otherwise the build won't have a chance to be finished successfully.